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.

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 and clean the optical surfaces, such as the front lens of the eye piece, in a circular manner. Lens paper should not be used, as it may scratch 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.

Vascular tissue of a pumpkin plant.

Here is a list of common mistakes which I observed over the years:

• Viewing specimens without a cover slip: The objectives are designed to be used with a cover slip. If no cover slip is used (or no water beneath the cover slip and the slide), then the focal distance will change and the quality of the image is reduced as well.
• Using immersion oil with a non-immersion objective: Lower image quality and dirty optics are the consequence. The oil, if not properly cleaned, will start to accumulate dust and image quality may decrease to the extent that no image is visible at all. Use an alcohol:ether mixture and lens paper to clean the objectives, but make sure that the solvent does not contact the lens too long. Otherwise the lens kit holding the lens in place may start to become soft.
• Using the coarse focus with higher magnification objectives: This may result in crashing the objective into the slide. Spring-loaded objectives offer a level of security here.
• Turning the fine focus adjustment for a long time to find a focus: This too may result in crashing the (high-power) objective into the slide. Instruct the students to restart their observation with the low power objective.
• Using the iris diaphragm as a means to control the amount of light: The iris diaphragm of the condenser is there to regulate resolution and contrast, but not to regulate the amount of light. At high magnifications it may be necessary to open the diaphragm to produce a brighter image, but the students should first use the dimmer to control the light.
• Switching the microscope on and off with the dimmer set to the highest light intensity: The lamp is heated up quickly. It is better to slowly increase the light intensity with the dimmer.
• Starting to observe with a high magnification objective: This is a common thing to observe. Students should start with the lower magnifications first. This allows them to select the area of interest of the specimen.
• Using thick, non-translucent specimens: For specimens of these types, it is better to use a stereo (binocular-) microscope.
• Using oil-immersion objectives without oil: This changes the focal distance of the objective and results in a low quality image. Students may then turn the focus knob to the extent of crashing the slide into the objective.
• Moving the microscope with the lamp switched on: This may result in a lower lamp lifetime. Move the microscope only when the lamp is cold.

Cross section of the stem of a sunflower.

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.