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.

Macro image of the front lens of a dirty and cracked 40x objective.

A clean 40x objective provides a sharp and crisp image.

A dirty objective produces soft, low-contrasty images. The picture was taken with the above 40x objective.

Most devices were still in a reasonably good condition, with the biggest problems in the mechanics. A check of the optics revealed that most of them were still quite OK, but the 40x objective of one of the scopes was in a particular desolate condition. A macro image of the front lens can be seen on the right. I suspect highly that one of two things happened to the objective:

The objective could have been rotated into immersion oil and was subsequently not cleaned. Students sometimes want to use a lower magnification after they used the 100x oil immersion objective.

A second possibility is, that the “dirt” on the objective is in reality resin for making a permanent slide. Maybe some students attempted to make a permanent slide and used too much resin, and did not wait for the resin to dry out. The front part of the objective was then rotated into the resin.

The origin of the crack in the lens, remains a mystery. The lens is spring loaded , and retracts when crashed into the specimen.

The resulting image was not usable at all. I included two pictures of the same area, one with an intact 40x and one with the dirty and cracked objective from above. I think that the two images speak for themselves.

What do we learn from this? Proper microscope instruction saves money. And be really careful about using immersion oil and resin in the classroom. Don’t even get the students into the position of making such mistakes. In my view, a 100x oil immersion objective is not even necessary for most microscopic work (unless you deliberately want to teach the students different microscopic techniques). Remove the objectives from the microscopes and store them in a safe place.

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.

Unplug the microscope. Open the bottom of the microscope to reach the lamp. Pull the lamp out and replace it with a new one. Never touch a new lamp with your fingers. Careful, the lamp may be hot!

A microscope lamp will last for many years. Tungsten lamps have the disadvantage that the color of the light will shift towards the red end of the spectrum and it may be necessary to exchange the lamp even before it burns out. Digital cameras are sensitive to the red end of the spectrum. This shift in color can easily be compensated by using a blue filter (“daylight filter”), but I have also seen older lamps which have a very pronounced red component.

When replacing a lamp, follow the instructions of the manufacturer. The following instructions are for microscopes that have a lamp compartment accessible from the bottom.

• Disconnect the microscope from the mains. It may also be a good idea to wait some time for the transformer to lose its magnetism (danger of electroshock otherwise? I don’t know, never tried it out… Energy is stored in the magnetic field of the transformer.)
• Turn the microscope to its side, taking care that the eyepieces do not fall out.
• Open the lamp compartment on the bottom of the microscope. Flip out the lamp.
• Remove the lamp and replace it with a new one. Do not touch the new lamp with your fingers, wear gloves or use a piece of cloth. Fat deposits will burn into the lamp and result in a darker image.
• Close the lamp compartment.

Lens paper, cotton swabs and an appropriate cleaning solution.

Darkfield microscopy is very sensitive to dust. Notice the bright spots (dust) on the dark background. This dust is not visible in bright field.

The following liquids can be used for cleaning:

• Water: Use water to remove dust from the body, stage, lamp (non-optical parts) of the microscope. Moisten a piece of lint-free cloth and remove the dust. Be careful when cleaning non-metal (plastic) parts, they may be scratched.
• Ether:alcohol (80:20 or 70:30, depending on manufactuer): Moisten a cotton swab or lens paper and clean the optical surfaces. The ether makes the solvent volatile, there is not much time for the cleaning solution to adversely affect the optical coatings and/or the lens kit.
• Cleaning fluid recommended by the manufacturer: Use as specified!

Do not use:

• Cleaning fluids made for computer screens: What do they contain? How aggressive (or not) are they? You do not want to risk the removal of the optical coating of the lenses.
• Cleaning fluids made for eye glasses: They too may contain substances that harm the coating of the lenses.
• Tissue paper or cloth made for eye glasses: they may contain additives that could scratch the optics. Use lens paper instead.
• Compressed air: Many compressed air cartridges for electronics contain additives that will form a milky layer on the optical surfaces. There is also the danger that you will frost the glass (expanding gasses have a cooling effect) and the rapid cooling may not be the best for the lenses. Don’t use them on optical surfaces.
• Other solvents: Acetone, xylol, etc. They may either attack the plastic parts and/or may dissolve the kit holding the lenses in place.
• Cotton swabs made for medical uses: they may contain disinfectants or other additives.
• Denatured or rubbing alcohol: Additives in the alcohol make the alcohol taste bitter. These additives also form a smear on the optical surfaces.

Cleaning the different parts:

• Cleaning the Eyepiece: First blow away the dust. Dust is able to scratch the surface. Then take a cotton swabs (real cotton, not artificial fibers). Moisten the swab with the appropriate cleaning fluid and make sure that there is no excessive liquid on the swab. Clean the eyepiece with the moistened swab. Then wipe the lens dry using lens paper. Do not apply too much pressure.
• Cleaning the Objectives: Immersion oil is first removed by carefully (no pressure!) wiping the objective with lens paper. You can then use the cleaning fluid recommended by the manufacturer. Do not dip the objective into the solvent (this should be obvious, but you never know what some folks are up to…). Apply the solvent to the lens paper.
• Cleaning the Microscope Stage: This can be done with a moist cloth dipped in an appropriate cleaning solution. Scratching of the stage should not be a problem (it’s made of metal), but do take care of the condenser lens!
• Cleaning the Microscope Body: The body can be cleaned either with a soft cloth and water, or dust can be removed by using pressured air (not those for electronics, which contain additives!)
• Cleaning the Condenser: The same things apply as for the objective and the eyepiece. First remove the dust, then use an appropriate cleaning solution with cotton swab and wipe dry with lens paper.

The effects of dust: Store the microscope in a dust-free environment and/or cover the microscope with a dust cover when not in use. Dust has several adverse effects:

• It disturbs the quality of the image. Especially dark-field illumination is very sensitive to dust. Even the smallest dust grains show up.
• Over the years the oil in the gearing (focus knobs, condenser knob, and of the mechanical stage, etc.) will collect dust and start to solidify, making the mechanics difficult to operate and increasing wear.
• Dust grains located between the condenser and the slide may scratch the condenser lens when moving the slide. The distance between the slide and the condenser lens is very small if the condenser is moved into the highest position.

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.

Microscopes generally do not require much maintenance and should deliver many years of reliable performance if kept well. Improper use of the instruments causes the instruments to age much quicker. Course work can be quite tough on the devices and a regular functionality check prolongs the life of the instrument.

• Cleaning the eye pieces of fat: Fat from the eye lashed accumulates on the surface of the eye piece. Use a cleaning fluid and cleaning cloth that does not damage the optical coating of the optics.
• Cleaning the objectives from immersion oil: Use synthetic immersion oil only! Natural oils may solidify over time. Even if synthetic oils are used, make sure that the students clean the objectives properly after usage. Also check the non-immersion objectives for the presence of immersion oil! Occasionally students do rotate non-immersion objectives into the oil. Use a cleaning fluid that does not harm the cement that is responsible of holding the lens in place!
• Checking the working of the gears and cleaning the gears from hardened oil: It is not good to turn the knobs by force. This may increase the wear of the gears over time. The gears should be cleaned of solidified oil so that all of the knobs turn smoothly again.
• Replacing old light bulbs: Old light bulbs start to shift towards the red end of the spectrum. A blue filter (daylight filter) restores a more balanced color, it is best to replace old bulbs even if they are still functional. When replacing bulbs, do not touch the new bulb with your fingers, as the fat of your fingers will burn on the bulb and decrease the light intensity.