The Microscope and Its Use
The microscope plays a very important role in
laboratory for the tissues and organisms which are too small to be seen clearly with the naked eye. With the aid of the microscope, however, these objects may be magnified so that an entirely new world of life is opened to our vision. Such an instrument, is of necessity, very delicate and built to high precision, hence it must be handled and used with care and understanding in order to obtain the desired results and to avoid serious damage to the microscope. Please keep these facts in your mind before you use your microscope. Even though you have used a microscope before it is quite likely that there are many points about its use which you have forgotten, or never knew, and you should make the same careful study about its use as the others who have never touch a microscope before.
In order to understand the proper use of a microscope we must first learn the parts that make it up and function of each. The drawing of a microscope giving here will serve a guide to the identification of the parts. Learn to identify all of these parts and to know them by names in the directions to follow and you cannot understand the directions if you do not know the names of the parts.
There are several points should also be noticed:
»» 1) The usual class microscope has eyepieces/oculars magnifying x 10, and an objective nosepiece carrying x 4, x 10, x 95 (or x 100) (oil immersion) lenses. Normally the three lower-power lenses are mounted on the nosepieces, whilst the oil immersion objective may be mounted or kept separately.
»» 2) Every time it is used, the microscope should be set up to the best optical advantage.
»» 3) Keep in mind the limit to resolution.
»» 4) The section has some thickness, so that the fine-focusing adjustment should be used continually during observation to bring out fine detail, e.g., cilia on cells. Essentially, though, we are getting a two-dimensional picture from an originally three-dimensional piece of material. For what the structure looked like in the third dimension, you can try to reconstruct mentally what is going in the missing dimension, and look up views of the structure in scanning electron microscopy.
»» 5) Artifacts (appearances not due to the original nature of the material as obtained from the body) can arise at all stages in the treatment of the section. Gross examples arise from:
clumsy excision from the body;
(b) poor or inappropriate fixation;
(c) shrinkage and worse, uneven shrinkage leading
to artificial spaces and distorted
(d) cutting scores or chatters from a bad
(e) section not flat on the slide;
(f) water, dirt or bubbles on in the
(g) dirt on the microscope lenses;
(h) patchy or faded staining, unbalanced staining
when more than one stain has been applied;
(i) precipitate from fixative or stain;
(j) tears and folds in the section.
»» 6) Setting up the microscope
|(a) Adjustment of light by using artificial or natural light.
(b) Light intensity can be increased by bring the lamp nearer to the mirror, if the lamp is not built-in.
(c) If the condenser is in use (nearly always), use the plane side of the mirror to reflect light accurately up the optical axis of the microscope.
(d) Raise the condenser to very near the underside of the stage, and open the iris diaphragm.
(e) Place a clean, stained slide on the stage and using the coarse and fine adjustment head bring it into focus.
(f) The iris diaphragm should now be closed just to the point where glare is eliminated. Further closure will make the field too dark and reduce resolving power.
(g) The microscope is now set up for use, but the requirements change for each objective. Higher power objectives require more light, thus the iris will need to be opened and perhaps the lamp brought nearer to the mirror and condenser re-fused.
(h) Note that the objective lenses are of different lengths, and they are not always par-focal. Be careful when switching in a higher power lens that it does not hit the slide because of its greater length. Clean the lenses only with lens paper.
»» 7) Some final suggestions about the use of the microscope:
|(a) About eyeglasses. Many students wear glasses and may wonder whether they should keep then on or take them off when viewing objects through the microscope. It the trouble is due to simple near- sightedness or far-sightedness there is no need to wear glasses. They may be worn, but a slight change in focus will be necessary. If the student has astigmatism, it is desirable to wear the glasses while using the microscope since the microscope lenses will not correct for astigmatism.
(b) Keep both eyes open while using the microscope. the beginning student may find it tiring to hold one eye closed while looking through the eyepiece with the other. Sometimes one may be observed holding his hand over one eye to relieve this strain. It is possible to see clearly by keep both eyes open. Through practice you soon learn to ignore that which is seen by the eye not over the eyepiece.
(c) Never try to find an object under high power if you cannot find it under low power. Often students will be observed searching vainly for an object under high power. When asked what the trouble is they will reply, " Well, I couldn't find it under low power, so I though I would try high power." High power is much more difficult to use than low power, then rest assured you will not find it under high power.
(d) When preparing to put the microscope away for the day, always leave it with the low power under the tube and with the diaphragm wide open. There is danger that the high power lenses may be run down through the open in the stage and damage the condenser or the iris diaphragm is left partially closed. this cannot happen if the lower power is under the tube and the diaphragm is wide open.
(e) High power should never be used with a cover glass.