Parts of the following are good practice with any microscope, it’s hoped those who find the rest dull will know the difference. -K
In preparation for a little time at the microscope one should first set their table in order. If more then one ocular is possessed it should be brought out, together with the specimens to be examined. One should have at hand a paper and pen, on the chance that a note or a sketch will need to be made. A lamp should be on the table as well, a simple desk lamp with a whitened bulb will do (no bulb with a visible filament will do with a student microscope). If it does not command the table when not in use, the microscope should be brought out or otherwise uncovered and given a quick dusting to remove any that might have settled.
Whether one chooses to work seated or standing, the height of the table (and stool) should be modified for comfort while working. Because one is unlikely to spend considerable time at the eyepiece if uncomfortable, it is important to incline the microscope and arrange the table specifically for comfort. An arrangement that seems comfortable yet results in an ache after prolonged use should be modified in the future. Do not persist with an uncomfortable set up out of convenience or stubbornness. In the below photograph one may be startled to see the closeness of the microscopes foot to the tables edge. This arrangement is that found most comfortable for the writer; with only the writing hand in position to rest upon the table, no stiffness develops across the users shoulders even after several hours.
When the lamp and microscope are in readiness one may turn on the light and begin orienting the illumination. When working without a condenser only the concave mirror will provide plentiful light from a convenient source∗. The distance from the lamp to the mirror is not critically important and will be suitable from 10 to 15 inches(25 to 40mm) provided that an excessively low powered objective is not in use. If one makes use of a 48mm objective (for example) it will be necessary to move the lamp closer to the microscope so that an evenly lit field will fill the eyepiece. Smaller or larger size bulbs than are standard will require the lamp to be closer or farther.
Placing a slide upon the stage is a simple thing when no mechanical stage is in use. It is surprising then that so many go about it in a manner liable to damage both slide and spring clips! A slide should be slid into the open space next to the posts by which each spring clip is held to the stage. Once there it is gently pushed up into the area where the clips contact the stage. The slide is now held firmly in place and no undue tension or stress has been or will be placed on either slide or clips. It should never† be necessary to pry up the end of a spring clip.
With the slide in place one should look askance at the stage and objective while racking down to within the working distance of the objective. The student (and an astonishing number of very advanced users) would do well to remember that the equivalent focus marked on the objective is not the working distance‡. As a general rule one may bring the objective as close as one may with the coarse focus when observing from the side, even when it is known to be too near. Upon next shifting the eye then to the ocular one may rack up with the coarse focus confident that the proper focus will be found.
Once the microscope is focus in a general way certain aspects of contrast will have to be addressed. Remove the eyepiece and with the eye located ten inches from the tube sight down the body of the microscope. The user should then select the diaphragm aperture that obscures the outer third of the brightly lit rear lens of the objective in use. Some sources will recommend the obstruction of only a quarter or as much as two thirds of the objectives rear lens. Although in many respects the proper aperture for the best resolution and contrast is dependent on the optical system, when using a concave mirror (and of course no condenser) it is less critical.
Proper lighting being now secured the user may replace the ocular and set to work in earnest. The slide may be moved about as required and the fine focus may be continually manipulated one way and the other to provide a better understanding of the observed structure. Should observation with the second objective be required only the fine focus will require manipulation to find focus however, one should always observer from the side when moving from a lower to a higher power. With any change of objective it will also be necessary to again determine the correct aperture§ to employ as described above.
∗In theory, parallel rays of light traveling from an infinite distance strike the mirror and are focused into a cone of light that converges in the image plane of the specimen. One then may imagine that a ground glass (or paper screen) might be placed in the path of that cone and raised or lowered to determine the focus of the concave mirror. This is only effective as an exercise if one is using daylight (genuinely parallel rays of light) as their light source and the mirror in question is parabolic rather than spherical. Because the mirror is apt to be a concave spherical section, the light will not focus to a single point. Those who love geometry might like to calculate some conic sections…
†Even when using slips of non-standard size it is advisable to employ a variation of this method (or employ larger stage clips) rather than bend and potentially deform the clips or create a situation in which they may spring down upon a slide. For a long while it was my practice to forgo the use of spring clips entirely when using the microscope vertically, after much manual micrography I am now so much in the habit of leaving them in place that the absence of spring clips is intolerable. I would recommend any newcomers to use spring clips whenever present and avoid the little frustrations of jostled slides.
‡So very much effects the working distance that it is often only looked at in a very general way, this is no exception! Second only to magnifying power, (higher magnification equals shorter working distance, how much shorter relates to how that higher magnification is obtained) working distance is closely related to numerical aperture. A 10x objective of 16mm equivalent focus having a numerical aperture or 0.25 will have a shorter working distance than a similar objective with a lower numerical aperture. Remember if one wants higher resolution, one must pay the price for it, both financially and with a shorter working distance.
§If one always employs a similar distance to the lamp and inclination of the microscope the optimum aperture will remain the same for each objective and sighting down the tube with the ocular removed will be unnecessary. For this reason alone a disc diaphragm should be considered a positive feature. The is no significant gain in ability to be had by having an iris diaphragm microscope without a condenser.