Problems with the optics

• Dirty lens: This is due to immersion oil on the optical surfaces, which have collected dust and have hardened.
• Lens kit dissolving: Some lenses are glued together. Flower-like bubbles forming in the lens are an indication that the lens kit is coming loose.
• Fungi on the optical surfaces: This is a problem with microscopes which have been in use in areas of high humidity (such as the tropics). An anti-fungal coating of the lenses may prevent this.
• Scratches or cracks: These can occur if the objective is rotated into the specimen. You can see an extreme example of this in the following post: Dirty microscope objective: Its effect on image quality
• Loss of coating: Excessive rubbing or a wrong cleaning solution may remove the anti-reflective coating of the lenses.

Problems with the mechanics

• Stage drift: In this case, the stage slowly lowers due to its own weight. This can be fixed by tightening some screws.
  Focus difficult to turn: In this case the oil in the gears has solidified due to age and accumulated dust. Do not use force, it may increase the wear on the gears. It’s better to get the device cleaned.
• Mechanical stage difficult to move: Like with the focus knobs a solidified oil makes the stage difficult to move.
• Too much slack: Sometimes there is too much tolerance and turning the focus knobs. There may be too much slack in the gears, possibly due to too much wear.

Problems with electricity

• Old lamp: An old lamp will have a spectrum shifted towards the red. This is a disadvantage for digital photography. The sensors of the camera are very red-sensitive. Use a blue filter.
• Brittle insulation: Old power cables may become brittle and be a hazard.

Rule 1: Be weary about “department store” microscopes

Enthusiasts who want to pick up the hobby frequently encounter their first microscopes in department stores and toy shops. If you are serious about microscopy as a hobby, then I have to disadvise you from purchasing these devices. Microscopes are precision technical instruments and the low cost of toy microscopes simply does not allow them to keep up with the demands of the more serious enthusiast. The resolution of the optics is lower. Stability can also be an isue. It’s better to invest a bit more. You have to contact a retailer which is specialized for microscopes and who sells microscopes to hospitals, schools or research organizations.

Rule 2: Consider carefully if you want a stereo microscope or a compound microscope

Consider your areas of applications. Do you want to observe large or opaque specimens (stereo microscope) or are you more interested in observing small, transparent objects (compound microscope). If you want to do microscopy work with young children, then I would recommend stereo microscopes. See the other post for more info: Which Microscope for Children?. Compound microscopes allow you to observe much smaller specimens, but require you to engage in sample preparation (unless you purchase ready-made specimens).

Rule 3: The magnification is one of the least important criteria

Resolution, stability, extensibility, light intensity etc. also play a significant role. Get the big picture and look at the whole device. Do not get bogged down simply on magnification. Getting a high magnification is the easiest thing to achieve. Simply add a stronger eyepiece, or take a picture and enlarge it on the monitor. Magnification without resolution is meaningless. And a shaky plastic microscope will produce such an unsteady picture that you won’t be able to see much anyway.

Rule 4: Go for standards

Make sure that the microscope has exchangeable objective lenses manufactured according to the “160mm” standard. In this case you have a wide selection of different objectives available from different manufacturers. Infinity corrected optics are an alternative, but there is no universal standard. Some microscopes are not modular in design (“closed system”) and it is not possible to exchange parts later on. When choosing the microscope make sure that you also consider possible future interests and uses.

Rule 5: Consider your current interests

Microscopy does not have to be an entirely new hobby, it can also be a valuable extension of one of your existing pastimes. You may want to evaluate your current hobbies to see which type of microscope fits best.

• Choose a stereo microscope if you are collecting stamps, minerals, rocks, coins, trading cards, smaller antiquities, insects or other objects that are small enough to be placed directly on the stage. Also choose a stereo microscope if younger children should have access to the device.
• Choose a compound microscope of you are keeping a home aquarium, if you want to make specimen preparation (microtoming, staining, etc.) as part of your hobby.

Bottom LED light of a stereo microscope. A single LED is sufficient. The glass stage plate has been removed.

Top LED light of a stereo microscope.

Rechargeable batteries deliver an operation time of approx. 50 hours with one charge.

• Long life span: LEDs have a life span of approximately 50 000-100 000 hours. A microscope which is in operation an unrealistic 12 hours per day would have a life span of 20 years+. Unlike tungsten or halogen light bulbs, LED lamps do not need to be replaced and are practically maintenance free.
• Low energy use: The electrical power is converted 80% into light, with nearly no heat production. The efficiency is therefore very high. This makes it possible to operate microscopes with batteries and without a power supply. LED microscopes can therefore also be used in classrooms in which the tables do not have an electrical power outlet. Incandescent light bulbs, in contrast, only convert 20% of the electrical energy to light, the rest is lost as heat.
• Not sensitive to movement: Hot tungsten or halogen light sources should not be moved during operation and when they are hot. Movement reduces the lifespan of the bulb becasue a hot filament breaks more easily when exposed to shock. LEDs are insensitive to movement. Bumping the microscope when the LEDs are on will have no detrimental effect on the LED.
• Can be switched on at full power: Incandescent light bulbs should not be switched on with the light intensity setting at maximum level. Rather, one should slowly increase the intensity after turning the light on. Sending maximum current through a cold bulb will reduce its lifespan. LEDs are insensitive in this respect. Sending a current through
• No color drift with age: LEDs do not significantly change their color or intensity with increasing age. Traditional incandescent light bulbs will shift towards the red end of the spectrum as they age. This is because over time a thin matallic layer will deposit on the inside of the glass bulb. A slight color shift can also be observed with LEDs, this is due to the aging of the resin of the LED.
• Nearly no heat produced: LEDs produce a cool light. LEDs pose no danger of overheating the specimen. This is important when observing water samples with live organisms. A heated specimen will drive out oxygen from the water sample which may possibly reduce movement of some (heterotrophic) organisms.

The following microscope features are probably not needed (or irrelevant) for introductory microscopic investigations in schools. These are the places where one can save money.

• Infinity optics: They are expensive and not yet very common for lower priced educational microscopes. Infinity optics are not interchangeable between manufacturers – to my knowledge there is no single world-wide standard. This can become problematic if individual objectives have to be exchanged, as one is bound to a particular manufacturer. The advantage of infinity optics becomes evident when working with filters or when doing photographic work. For this reason they are commonly employed in research-grade biomedical microscopes. Stay with the 160mm tube length standard objectives.
• 100x oil-immersion objective: This objective requires more experience. The danger is, that students confuse the oil-immersion objective with the regular air-objectives. There is the danger of contaminating these objectives with the immersion oil. Once the specimen slide is covered with oil, it is not possible to use lower magnification objectives anyway, without significant loss of quality. The oil may also stain the sticker of permanent slides. Using oil immersion objectives without oil is also not recommended. The image quality will be very low and students may rotate the objective into the specimen slide while searching for the best focus. If the oil is not properly cleaned, then dust will accumulate over time further reducing image quality. Choose a 100x oil objective if it is an educational goal to teach students to properly use this type of objective or if you need the high magnification (such as for observing chromosomes in cells).
• Phase contrast optics: Generally they are more expensive. These optics will distort the natural color of the specimen as it converts differences in refractive index into difference of brightness. They are useful for investigating unstained bacteria, and it is questionable if this type of investigation is suitable for introductory microscopic courses.
• Köhler illumination: This illumination system is useful when doing photographic work. For students it poses an additional diaphragm to worry about. The system has to be properly adjusted and aligned before use.
• Tension-free objectives: They are used for polarization work. They are expensive and probably they won’t sell them to you anyway if you tell them that you need the microscope for educational or amateur purposes. I’m just mentioning this for the sake of completion.
• Apochromatic objectives: These objectives are corrected for chromatic aberration and are primarily used for photographic work where high image quality is needed. Achromatic objectives are standard for educational work.
• Plan objectives: These objectives deliver an image which is in focus from center to edge. They are commonly used for photomicrographic work. They are expensive and deliver no significant advantages for educational work.

A variety of different standards exist:

• Standard slide: 26 x 76 mm
• Geological slide: 75 x 50 mm
• Petrographic slide: 46 x 27 mm
• Thin sections slide: 48 x 28 mm

Microscope glass slides may be modified in a variety of ways:

• They may have a central indentation to carry several drops of liquid.
• They may have a frosted side to allow for easier writing with a marker.
• They may have polished corners to reduce the possibility of injury due to sharp corners.

Cover glasses (cover slips) exist in a wide range of different sizes, square, round, rectangular. Common sizes include:

• 18x18mm
• 20x20mm
• 22x22mm
• 24x24mm
• various rectangular sizes up to 24x60mm to cover nearly the whole slide.

Choose a cover glass that corresponds to the size of the specimen and the slide. The thickness of the cover glass is important, as it has a significant impact on the resolution of the image. The thickness should correspond to the thickness indicated on the objective lens. In many cases, the cover glass is 0.17mm thick, but there is often a small variation even in the same batch. For critical purposes, it may be necessary to measure the thickness of the individual cover glasses to find one close to the desired thickness (use a vernier caliper to determine the thickness).

Light microscopes (optical microscopes) that are commonly used in schools come in two flavors – compound microscopes and stereo microscopes (also known as dissecting or binocular microscopes). In research or medicine the range of optical microscopes is naturally larger, some of which are variations or adaptations of the above two types. Strictly speaking, Laser Scanning Confocal Microscopes also belong to the category of light microscopes, but for educational purposes in schools, they are not relevant.

• Compound microscopes: When confronted with the term “microscope”, most people will have a picture of a compound microscope in their head. Compound microscopes are used to observe small, thin, translucent objects. In many cases it is necessary to prepare the specimen before it can be observed.
• Stereo microscopes: These microscopes are designed to view larger, opaque objects. Translucent objects can also be viewed. The magnification is lower than in compound microscopes, but with the advantage of a stereoscopic view.

Important: Compound microscopes with a binocular head should not be confused with stereo microscopes. Compound microscopes are not capable of delivering a stereoscopic (3D) image, even if they have a binocular head.

I recommend the following accessories for each microscopic work place:

1. Tweezers: They are useful for placing the cover slip on the specimen and for picking up small specimens (insects, thin cuts, etc.).
2. Dropper: For placing a water drop between the slide and cover glass.
3. Scalpel: Useful for cutting away not needed plant tissue or algae. Do not include for young children.
4. Watch glass: For storing water for making temporary slides.
5. Slides: There are several types available. Some have a frosted side to allow for easier writing, others have rounded edges to decrease the possibility for injury.
6. Blue filter: Useful for compensating the red tint of old tungsten lamps.
7. Cover glasses: obtain those that correspond to the objectives. 0.17mm thickness is standard.
8. Needle: Useful for separating algae or to pick up very small samples of material to be observed.
9. Scissors: For cutting filter paper to remove excess water.
10. Small petri dish: For the storage of specimens that need to be kept in water (plant material, algae, pond water etc.). Cuts of plant material are stored in the dish before they are observed.
11. Plastic tray: For storing the above accessories.

The following accessories are also commonly used, but may not be recommended or necessary for each individual workplace. Safety is an issue as well!

• Razor blades: for making cuts through leaves and stems. Too dangerous to be stored in every workplace, and not always needed.
• Stains: Some stains are toxic, especially those that are used to stain the DNA inside the nucleus of cells (possibly carcinogenic!). Some may stain clothing irreversibly.
• Mounting media: These are used to make permanent mounts. They may contain organic solvents which are not healthy when inhaled. There is also the danger that students confuse them with the immersion oil…..
• Eldermarrow, styrodur or styrofoam: These are used to make thin cuts of plant material. The plant material is squeezed between two layers of this material and then cut. Eldermarrow is recommended. Styrodur and styrofoam also work but they are very tough on the razor blades and will make them dull extremely quickly.

What do the numbers and abbreviations on an objective mean? Especially when buying used microscopes from research laboratories or hospitals a basic knowledge of the text written on the optics can become handy. You don’t want to buy things that you don’t need.

• A or ACHRO (depending on brand): This signifies that the objective is an achromat. This means that chromatic abberration was corrected for 2 colors (in contrast to the expensive APOchromatic lenses). Achromatic lenses are those most commonly found in education, they are the cheapest.
• PLAN: These objectives produce an image which is in focus from edge to edge. They are used for photographic work and are more expensive.
• PLANAPO: This refers to a planapochromatic objective. It produces a flat image (in focus from edge to edge) and it is has a chromatic abberration correction for 4 colors. Expensive and not needed for educational work.
• PLANFL: A Planfluorite objective. A bit less expensive than the planapochromats but also not as fully corrected.
• 160: This represents the standard tube length of 160mm. Objectives with this standard are interchangeable between manufacturers.
• 0.17: This represents the thickness of the cover slip to be used in mm. Coverslips with a deviating thickness will result is an image of lower resolution.
• 4, 10, 20, 40, 100: This represents the magnification of the objective. The total magnification is calculated by multiplying the magnification of the objective with the magnification of the ocular (eye piece), which is usually 10x. The magnification is also indicated by the ring colors:
  • red: 4x or 5x
  • yellow: 10x
  • green: 20x
  • blue: 40x, 50x or 60x
  • white: 100x
• OIL: This designates an oil immersion objectives. Do not immerse non-oil objectives into immersion oil!
• WI: Water Immersion. Here water is used instead of oil.
• 0.65 (etc): This is the numerical aperture. This value indicates the angle to which an objective is able to receive light. This value also determines the resolution of the system. For maximum resolution, the iris diaphragm should be set to a value equal or larger than the numerical aperture of the objective in use.
• NCG or NC: These abbreviations stand for “No cover glass”. These objectives are designed to be used without a cover glass. They are useful in the medical area where blood smears etc. are observed.
• LWD or ULWD: These abbreviations stand for “long working distance” or “ultra-long working distance”. These objectives are able to work with a large specimen-objective distance and are used for specific applications.
• P, POL or SF: These objectives are designed to be used for polarization microscopy. The objectives are strain-free (SF) and will therefore not modify the polarization of the light. They are not necessary for simple polarization microscopy conducted in classrooms.
• PL or NH: These are designation of objectives used for phase contrast microscopy. A PL (positive low) objective produces an image of a specimen which is darker than the background, a NH (negative high) objective produces an image which is brighter than the background.
• NIC or DIC: Nomarski Interference Contrast or Differential Interference Contrast objectives produce an image of a specimen which appears to be slightly 3 dimensional. If you use a filter to achieve oblique illumination, then the result will look similar.

Before buying a second hand microscope, take care of the following points. The list is certainly not complete, but should give an overview of the things to look out for:

• Appropriate optics: Make sure that the microscope is equipped with the objectives that are needed for your task. Many microscopes from hospitals and research institutes are equipped with phase contrast or plan apochomatic objectives. These are expensive and not required for educational work.
• Are the focusing knobs easy to turn or has the lubrication oil already solidified? Do not force-turn the focus knobs, it will increase the wear on the gears.
• Does the condenser move freely up and down, or has the lubrication oil already solidified? Does the condenser stay up, or is it pulled downwards by its own weight?
• Does the iris diaphragm open and close without problems, or has the lubrication oil already solidified?
• Were the non-oil immersion objectives dipped into immersion oil? They are not designed for this.
• If the objectives are spring-loaded, does the front part of the objective retract properly when pushed in? Or was the objective covered completely with immersion oil, now solidified?
• Are there any fungi growing on the lens optics? This may be a problem of microscopes used in warm and humid areas.
• Does the stage stay where it is, or does it move down due to its own weight?
• Does the microscope generally make a good impression?

Additional points to consider for schools:

• If you want to equip a whole classroom with used (or new) microscopes, make sure that they are from the same series. Students are then confronted with the same device each time and do not have to re-learn the peculiarities of each instrument – more productive lab work. It also makes makes it easier for the teacher to explain the handling of the microscope. If you buy used instruments, then you may not be able to obtain a whole classroom set at once (unless another school, college or hospital replaces its equipment at once).
• Make sure that the used microscopes have the appropriate optics installed and not specialized objectives! You need achromatic bright-field objectives. Many research microscopes come with phase contrast objectives installed, however. This type of optics also can be made to work like bright-field objectives but they are more expensive and you don’t want to spend money on things that you don’t need.
• In recent years the so-called infinity optics became increasingly popular, especially in research. Be careful – infinity objectives are not compatible with the “traditional” DIN 160mm systems, and infinity optics from different manufacturers are also not compatible with each other. When purchasing infinity systems (if you can afford them…), be aware that the optics can possibly not be exchanged with other microscopes that you or your school owns.

Occasionally people ask me for advice about which type of microscope to buy as a present for their children. I once responded to a newsgroup question making a very strong point in favor for stereo microscopes for young children (approx. 5 years of age), and I would like to reiterate these points below. Read the article “Different Types of Light Microscopes” for a description of similarities and differences between the different microscope types. The following section reflects my own personal opinion on this issue.

In any case, I do not recommend the purchase of “toy” microscopes. If you invest a little more you are able to obtain a “real” instrument with substantially better image quality and flexibility, one which will retain the interest of the child (and parent!!) for a longer time. And especially for children a good image quality is necessary. An experienced microscopist may be able to interpret the “dark washed-out blob” as a cell, but children need crisper and clearer images to maintain their fascination – my personal opinion. There is the danger of disappointment if they do not see similar images as those printed on the box, and I am almost certain that many “toy microscopes” are not capable of keeping their promise. But this is my personal opinion, and the quality of these devices certainly varies as well.

Some of these microscopes are also sold with unrealistic magnifications up to over 1000x. Please understand that toy microscopes are useless at this magnification, for a range of reasons:

• Resolution is too low: The object that you want to see is magnified 1000x for sure, but you only see a washed-out blob with no detail.
• Stability is low:. There is a good reason why microscopes are made of metal and why they are heavy. Every vibration (walking) is magnified as well and transferred to the microscope.
• Image is dark: A high magnification requires a high light intensity. Many of these microscopes are not capable of delivering the required light intensity.
• And: bad depth of field, optics not corrected for lens errors, etc. etc.

I have to admit that even “toy” microscopes vary greatly in quality (and price). If you want to buy one of these, then I would recommend you not to give magnifications above 200x or 400x much weight and to read appropriate reviews beforehand. A cheap plastic scope with 1000x magnification is unrealistic. I have already seen some better quality “toy” microscopes but the price difference to a microscope manufactured according to the international DIN standard was not too big. The bottom line is that the child should enjoy working with the instrument.

Compound or Stereo Microscope?

Instead of simply listing the pros and cons of each type, I’ll make life easy by giving you two simple rules:

The younger the child the more you should tend towards stereo microscopes.

and:

If you intend to purchase a compound microscope, make sure that it works with the DIN standard. This allows for an exchange of objectives and guarantees a minimum quality.

There are many points that speak for stereo microscopes for young children, they are not only more “child friendly”, and more forgiving and easier to handle:

• The subjective visual impression of 3D samples (flies, hair, rocks etc.) can be quite fascinating. The view is, in contrast to compound microscopes, upright (!). A big advantage for orientation.
• Little to no sample preparation required for many objects. There is no need to cut and slice the specimens into the required thickness, even though this is possible as well. There is no need to prepare specimen slides with cover-slips. We use stereo microscopes in our school, and as a first introduction, we gave our students a post card and made them look at the colored dots that compose the image – fast, simple with an immediate result.
• Stereo microscopes have a low magnification, often not more than 40x. This means that there is less abstraction “from the real world”. A fly looks like a fly, only much bigger and more impressive. For a compound microscope you need to take the fly apart first and examine the individual parts, it’s too thick otherwise to be observed.
• Stereo microscopes also allow for an observation of non-transparent objects like rocks, fingernails (the dirt is pretty interesting…), skin, plant leaves etc. Stick a whole earth worm under the microscope and see how it looks like. Directly observe a dish of pond-water. If the child is already collecting rocks, insects, stamps, coins, etc. then a stereo microscope is the natural extension to observe these collected items.
• Some stereo microscopes also allow for a change in magnification, by zooming. This is not a necessity, though.
• Decent stereo microscopes can be cheaper than compound microscopes, because they are less complex.
• Stereo microscopes need less time for instruction. More instruction time needed for compound microscope. With compound microscopes, if you use a higher magnification and then turn the coarse-focus-adjustment knob into the wrong direction, you run the risk of ruining both sample and objective because you smash the objective into the specimen. Stereo microscopes have a large sample-objective distance.
• Stereo microscopes have a higher depth of field. It is therefore much easier to find what you are looking for.

Stereo microscopes also possess certain disadvantages:

• There may be some samples that you or your child is interested in but requires a higher magnification. For example, if you want to watch the nucleus of cells, then you are better off with a compound microscope. It is also not possible to observe bacteria, they are simply too small. As a comfort, a compound microscope with regular bright-field optics also does not allow you to view living bacteria, as they are transparent, you need expensive phase-contrast objectives (labs use them). Alternatively the bacteria have to be stained first, and then I doubt that novices will be able to recognize them as bacteria.
• Sample preparation may indeed be one of the activities that a child may be interested in, but stereo-microscopes do not require much preparation, while it is necessary in compound microscopes.
• Some younger children may have problems viewing through both eye-pieces (they can look through one of them if they want to).
• And possibly the biggest “problem”: There is the myth that microscopes have to magnify very much in order for the person to see much. Children may be disappointed if they hear that their stereo microscope only magnifies up to 40x, if their friend has a department store (“toy”) microscope which magnifies 1250x. This is where education comes into play – magnification is not everything, and a high magnification does not mean that one sees more, resolution also counts. I want to guarantee you that you won’t be able to see much at 1250x.

There are a range of different features that one should consider when purchasing a new microscope.

• Spring loaded objectives: Especially at high magnifications the working distance between the specimen and the objective can be the fraction of a millimeter. One careless rotation of the focus knob and it is possible to smash the objective into the specimen. This may result in the destruction of both the specimen (cheap) and the objective (expensive). In order to avoid such damage, manufacturers have introduced spring-loaded objectives. The lower part of the objective is flexibly installed and pushed in when contacting the specimen slide.
• Same series: If several microscopes are purchased, they should be of the same brand and make. This allows for an easier exchangeability of parts. Sooner or later different parts will have to be replaced and devices from the same manufacturer keep the costs down.
• Optics standards: In recent years large microscope manufacturers have migrated towards so called infinity-corrected objectives. Be aware that these objectives are not compatible with the finite 160mm tube-length standard that was introduced in the 19th century and has remained popular up to date. The infinity optics offer several advantages, many of which are probably not relevant for educational purposes. In any case, do not combine objectives of different manufacturers or infinity-corrected optics with a microscope using a finite-optics standard. Many cheaper microscopes still adhere to the finite-optics standard and this is still commonly found in educational microscopes.
• X/Y Stage: For labwork, give a preference to systems that are equipped with an X/Y Stage and not clips. Moving of the slide by hand exerts pressure on the stage and this can result in a loss of focus. An X/Y stage allows the movement of the slide with two rotating knobs. X/Y stages should be equipped with a scale that simplifies the finding of relevant specimen parts. The slides can then be labeled with the coordinates for students to directly find the part of interest. Clips are useful for smaller microscopes used in fieldwork.
• Light, no mirror: Avoid the purchase of instruments that rely on natural lighting and a mirror. Sooner or later students will aim the mirror directly at the sun. This can cause irreversible eye-damage. Artificial lighting also makes the device independent of natural lighting and can therefore also be used in the evening time.
• Parfocal objectives: Parfocal optics allows for a change in magnification without the necessity of much refocusing. Make sure that the objectives are designed to work with each other in this respect. Parfocality is not automatically guaranteed.
• Focus block: I have seen some systems where even the low power objectives could be crashed into the specimen by careless focusing. I suppose that this is because of different manufacturers of objectives and the microscope. The microscope was, so I suppose, designed to accommodate a wide range of third-party objectives and there was not physical focusing block built in. In this case I recommend that the school purchases microscopes that allows for a manual setting of a focus block.
• Magnifications: I recommend the following objectives: 4x, 10x, 40x and possibly a 100x oil immersion objective. Again, make sure that they are of the same series and designed to work together. A 100x oil immersion objective is useful when cell division events are to be observed, mostly relevant for older students.

Objectives can be classified as follows:

• Parfocal objectives: Parfocal optics allows for a change in magnification without much refocusing. Make sure that the objectives are designed to work with each other in this respect.
• Achromatic objectives: These are the most common and also the cheapest objectives. Chromatic aberration is corrected for two colors. When observing specimens of high contrast it is possible to see red and blue fringes. Achromatic objectives are perfectly sufficient for routine analysis and for educational purposes. They do not, however, possess the resolving power of the better corrected objectives. Some achromatic objectives also display a slight image distortion. Both chromatic aberration and distortion may be annoying when conducting photographic work, but do otherwise not disturb. Achromatic objectives do have other advantages that make them suitable for course work. They have a larger depth of field and the working-distance (the distance between the objective and the specimen) is larger as well. This makes focusing easier and reduces the chance of crashing the objective into the specimen.
• Apochrmatic objectives: These objectives are corrected for three colors. Fringes are not visible and the obtainable resolution is higher. The trade-off is a reduced working distance and smaller depth of field. These factors and a higher price make apochromatic objectives less suitable for course work.
• Plan objectives: These objectives are available for both achromatic and apochromatic versions. They contain additional lens elements that correct the distortions. The cost of these objectives is naturally higher. They are commonly used for photomicrographic work. Especially the planapochomatic objectives deliver images with no recognizable chromatic aberration and distortion.
• Fluorite objectives: Fluorite objections are composed of relatively few lens elements. For this reason the contrast is higher. These objectoves are applied in special areas such as fluorescence microscopy or fine structure research.
• Phase contrast objectives: The phase contrast technique allows for visualization of transparent and uncolored specimens. Unstained bacteria, for example, are very difficult to see using the bright-field technique, but are clearly visible in phase contrast. Phase contrast requires special objectives, however. Phase contrast objectives are available also as achromatic, apochomatic, and plan versions. The microscope itself must also be equipped with an appropriate filter system to use this technique. Phase contrast objectives can also be used for bright field work, but the image quality is lower. Due to the higher cost of phase contrast equipment I recommend that only one or 2 teacher’s microscopes are equipped with this system. These microscopes can then be coupled to a video system for the whole class to see. Before the purchase of the system, the teachers should clearly specify the type of observations that are to be conducted. If much living material is to be investigated – material that can not be easily stained – then phase contrast is preferable. If students are to conduct sample preparation and staining, then bright-field objectives are probably the better option.
• Oil Immersion Objectives: These objectives are commonly used for magnifications around 100x. A drop of immersion oil is placed on the slide and the objective is rotated directly into the oil. Immersion objectives increase the numeric aperture and thus the resolution. They are useful structures inside a cell, such as the chromosomes of dividing cells. In a school setting, oil immersion objectives are a mixed blessing. While they do allow the observation of various sub-cellular structures, significant drawbacks should not be overlooked. It can happen that students confuse the objectives and rotate non-immersion objectives into the oil. If not properly cleaned (a common problem when there is not enough time for clean up at the end of a lesson), then dust will accumulate on the objective lens delivering a blurry image in future session. Students may also attempt to use a high power oil objective without oil. In this case parfocality is not guaranteed anymore and there is the danger that the objecitve is crashed into the specimen. If oil immersion is used, then only synthetic oil should be used. Natural oils may have the tendency to solidify if not cleaned properly.
• Water immersion objectives: These are not commonly used in school educational settings. They increase resolution by immersing the objective into water and not synthetic oil.

In most cases the teacher will already be supplied with an appropriate working environment. In the case that the school is required to set up a lab, the following pages may serve as a help. One should not underestimate the costs of setting up a suitable microscopy environment. The following issues should be considered.

Availability of power plugs: It should be possible to plug in the microscopes into the AV outlet without the use of extension cords. Needless to say, tripping over power cables can be expensive on equipment and student health.

Place for students to put bags: Personal belongings of the students, such as school bags, should be stored away from the work space, but still be accessible. It may be possible to store the bags beneath the work table. The teacher has to be able to move freely between the work areas. I require the students to deposit their bags in the front of the classroom.

Stable tables: The talbes should be mounted on the floor. This not only reduces vibrations but also prevents accidents.

Storage place for microscopes: The instruments should be locked away in a dust-free environment. The instruments should be reachable without problems. There should also be a place to accomodate the accessory equiment.

Curtains: Direct sunlight makes observation difficult. If the teacher uses a videoprojector to present something, then curtains are also necessary.

The equipment of a lab with microscopes can be a costly affair. In order to avoid misinvestment, several important decisions that must be taken into consideration:

• The type of microscope: compound or stereo microscope
• The number of microscopes
• Curriculum
• The manufacturer of the body and optics
• The features of the microscope

The type of microscope: The decision between compound and stereo (dissecting) microscope is a central issue. I would make this decision based on the age or grade level of the students. I do not recommend the use of compound microscopes for very young students, for a variety of reasons. If primarily elementary school to lower middle school students are to use the equipment, it may be well worth considering to purchase stereo microscopes only. Compound microscopes demand more extensive sample preparation (unless purchased slides are used) and more experience in handling and are thus more suitable for older students. There are also paedagogical issues related to the choice of the type of microscope. Stereo microscopes supply a three-dimensional upright image at a lower magnification. Parts of the specimens that can also be seen using the naked eye can now be observed much larger. It is much easier for the students to imagine size relationships this way. Schools with sufficient budget purchase both types of microscopes.

The number of microscopes: How many devices should be purchased? This is not only a budget issue. The costs of microscopes can vary greatly and it is easily possible to obtain 20 cheap devices for the same amount of money as 5 expensive instruments. What is the class size? How many students should use one microscope? How much storage space is available? What are the curricular demands? This question can not be easily answered an a few lines. I would recommend one microscope per 2 students. For a large class of approximately 30 students, this would mean 15 instruments. If the school intends to purchase both stereo and dissecting microscopes, storage space for 30 instruments must be available.

Curriculum: What skills should the students learn? Are sample preparation, staining, making specimen cuts a part of the curriculum? If yes, then there is probably no way around a compound microscope. Should rocks, insects and every-day objects be observed? In this case it may be more suitable to obtain stereo microscopes.

The manufacturer of the body and optics: There are many manufacturers that supply schools with comparatively cheap devices. Often the optics are not manufactured by these companies but purchased from specialized firms and included in the set. In my view the brand of the device is of minor importance. Available servicing is a much more important factor to consider. While microscopes generally do not require much maintenance, it may be necessary to readjust some of the parts periodically. Over the years the oil becomes thick and the focussing knobs become difficult to turn. A firm that is willing to perform the routine maintenance makes life for the teacher easier (unless the teacher him/herself is capable of maintaining the equipement).

When purchasing instruments, look out for the following:

• Stability: Make sure that the construction of the decive makes a solid impression. A solid device minimizes vibrations. Do not forget that even vibrations are amplified by the magnification.
• Mechanics: The focus knobs and X/Y knobs should turn easily and without friction. Is there any play when turning the knobs? Does the stage stay in place or does it shift? If the stage changes position due to gravity, then a readjustment is necessary. Does the revolving nosepiece center the objecitves properly? Is there any play in the nose-piece?
• Optics: Is the resolution and the contrast of the objecitves sufficient? Also make sure that the objectives are of the same series so that parfocality is guaranteed. Is the field-of-view of the eye piece large enough?
• Lighting system: Does the light intensity correspond to the objecitves? A higher light intensity is required for higher magnifications. A halogen lamp delivers a brighter image. This is an advantage if students are required to work much with the 100x objecitve.
• Student Proofing: A student proofed microscope requires special tools for the removal of eyepieces and other components. There are no finger-screws to turn, everything must be adjusted using the tools.

Notice that I did not include maximum magnification as one criterion. This is the least important aspect. In most circumstances a total maximum magnification of 400x is absolutely sufficient.

If your school decides to purchase used microscopes – hospitals and research institutions occasionally replace their equipment – then take care of the following points as well:

• Are the knobs difficult to turn? This is an indication that the lubrication oil has started to solidify and must be replaced. Do not turn the knobs with force, as this may damage the gearing.
• Check if the mechanics demonstrate excessive play. Over time there is a natural wear of the gearing.
• If dust is visible then the optics must be cleaned with pressurized air.
• Microscopes that were used in a humid environment may start to grow fungi on the optical surfaces. Some optics are sprecially treated with anti-fungal substances to prevent this from happening.