How to Work With Light Microscope

How to Work with a Light Microscope.

How to Work with a Light Microscope

It is now supposed that the instrument is ready for work. To start, it is well for the beginner to provide a few prepared specimens, as these will help him considerably if it is his intention, as it should be, to prepare them later himself. Whatever branch of study he is going to follow, a slide of pleurosigma angulatum, dry, will be valuable to practice upon and to determine the quality of his higher power objective. In this latter respect, however, the writer would advise the beginner to guard against expressing an opinion too soon. He knows of many cases where the optician's claims were flatly denied, when often a few words of advice by lettter or a few minutes of intelligent manipulation would resolve the diatom, and would thereafter do it so easily that it became a wonder how it could possibly be avoided.

For a low power objective, the proboscis of a blowfly is probably the most suitable and at the same time most interesting object. Place this upon the stage, and, after getting it as close as possible to the center of the opening in it, focus by means of the coarse adjustment. If only a portion of it can be seen and if it is desired to see a larger surface, the length of tube may be contracted by means of the draw-tube.

In this case the object will be placed out of focus, and another adjustment becomes necessary. If a higher power is desired, the draw-tube may be extended. Observe whether the field is well illuminated, and if not, bear in mind what has been said about properly adjusting the mirror. If the object appears milky or the light is so intense as to be painful to the eye, which is of usual occurrence to the beginner, the diaphragm should be turned from one aperture to another until a marked difference is seen ; or, the plane mirror should be used. In this connection it is well to state that the above precautions should always be observed with low powers, unless the object is thick. Now use the micrometer screw and note carefully the beautiful structure which is opened to view. After sufficient time has been spent, upon this, the objective may be replaced by a higher power and the object by a slide of P. angulatum ; focus upon this, being mindful of the suggestions previously given, and do not fail to observe what has been said in regard to a well illuminated field.

If this does not bring out the lines, swing the mirror-bar from the central position into an oblique one, on the side opposite to that of the light and readjust the mirror ; in doing this grasp the ends of the mirrorfork between the thumb and middle finger and move the mirror by the first finger. If the field can not be evenly illuminated, it is evidence that the mirror is beyond the limit of angular aperture in the objective, and it must therefore be brought back until it is.

It must here also be noticed that if the diaphragm is still attached to the instrument and does not swing with the mirror, it may also be the means of cutting off light. By means of the micrometer screw carry the fine adjustment back and forth beyond the plane of the object and observe closely whether any lines can be distinguished. It is very probable that they will show ; but if not, the cause should be determined. It may be that the magnifying power is not sufficiently great, and in this case a higher power eyepiece should be used, or the cover glass may be more or less than the normal thickness, which would cause a spherical over or under-correction in the objective. In this case th'e lines would appear when the diatom is not in focus. If the objective is a non-adjustable one, the proper correction may be approximately reached by means of the draw-tube. If the lines appear over the plane of the object, it shows overcorrection, and the length of tube should then be decreased, or contrary when the lines show below or beyond the plane of the object. If the above directions have been followed, the lines cannot fail to be seen with a moderately good j or 4 inch objective ; but if they are not, the trial should be repeated. Again, be careful to have no obstruction between the course of rays from the mirror to the stage ; get good illumination on the object ; observe well ; keep the instrument in such a position that the object is not illuminated from any other direction than from the mirror.

When the diatoms are resolved in this manner, the lines will appear to be diagonal in some ; longitudinal or transverse in others, according to their position ; anft, if the resolution is very good, these lines will further resolve themselves in minute beads of a hexagonal form.

It will now be well to bring the mirror more nearly to a central position ; do this at intervals of about 10 degrees, and note the appearance at each decrease of obliquity. It will be found that as the mirrror approaches the optical axis the lines will appear to become more faint, and may disappear before central illumination is reached ; in this case it will be well to begin again. An endeavor should be made to make each attempt give better results than the preceding one. Repeated trials will not only impress the various phenomena upon the mind, but will cause a notable improvement in manipulative skill, and thus a better performance in the objective.

Until now we have assumed that transmitted light has been used. We will now suppose that the object is not sufficiently transparent to use this method ; the object is then said to be opaque, and requires a different procedure. . We will say that it is desired to examine an insect ; it may be attached to a slide, or, what is better still, may be fastened in a stage forceps as it may then be turned and viewed from all sides. The low power objective should again be attached ; after having been focused, it will be found that the light is insufficient to illuminatate it. The mirrorbar should now be swung upon its axis around the stage to a point above it, so it will be at an angle of about 45 degrees to its surface. If a lamp is used and is in the same position as when used with transmitted light, it is probable that the tube of the instrument will obstruct the light, and it is then well to move it toward the front.

Using the concave mirror adjust it so that the light will be concentrated upon the object, by watching it directly, and then observe through the tube. If it is not sufficiently illuminated continue to adjust the mirror ; also vary its distance from the object and swing the mirror-bar to a higher or lower point. It often occurs that, under the best conditions, the need of better illumination is felt ; in this case a bulls-eye condenser should be procured. It will be found that this will become a useful and perhaps necessary accessory in work outside of this. Place it close to the instrument and set the bulls-eye between the object and source of light, with the plane side toward the object ; if an ordinary hand lamp is used, it will be necessary to elevate this to about the height of the eye-piece, and if it is to be used often in this position, a special support should be made for this purpose.

Low power objectives are usually used on opaque objects, but sometimes a higher power is desired. Unless one is constructed with a view to opaque illumination its working distance is usually so short that it will prevent the light from striking the object. A J or i objective, of 75 degrees, has sufficient workingdistance, and its mounting are made conical in the front, so that it will allow it.

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