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Information on illumination without using a substage condenser microscope.
Microscope Illumination Substage Condenser
Central Light. It has been necessary in the foregoing paragraphs to secure some light upon the specimen, but no directions have been given as to the proper illumination of the same. Accuracy of results depends upon correct illumination more than any other one thing. A vast majority of all microscopic work is done by light transmitted through transparent or semi-transparent objects. We will at present consider only such objects. The matter of illuminating opaque objects will be taken up later. The mirror is placed below the stage as a convenient means of reflecting the light through the object into the objective. It is plane on one side and concave on the other. The concave mirror is always used when the substage condenser is not used, except in the case of very low power objectives, when it is best to use the plane mirror. When the light is thrown upon the specimen and the objective focused as previously directed, remove the ocular and look into the tube at the back lens of the 16 objective. With the medium and higher power (i6mm and above) objectives the minimized image of the mirror with its mounting will be seen. Swing the mirror bar to the median line and as nearly as possible arrange the mirror so that its mounting will be concentric with the periphery of the back lens of the objective. All of the better microscopes are made with a ''center stop," indicating when the mirror bar is in a line parallel with the optical axis of the microscope. This is done because central, or axial, light gives a symmetrical illumination, which is best for observing the large proportion of transparent objects. This in itself does not insure axial light. The mirror must be so turned that the rays of light, or the axis of the cone of light, reflected from it enter the objective parallel to its axis.. This cannot always be done. Other considerations are more important than exact central light. In working with daylight, reflections from trees, window sash, etc., are apt to be seen on the mirror. If the whole microscope cannot be so shifted as to clear the mirror of these reflections the mirror itself should be turned so that, if possible, no images will appear upon it. If artificial light is used the mirror should be so turned that the image of the light is seen in the center of it. The more nearly this image covers the mirror, the better. If a bulks eye condenser is at hand, so place it between the light source and the object that a sharp image of the light source will be seen in the center of the back lens of the objective. If the above rules are followed it will be found upon replacing the eyepiece that the field is evenly illuminated. It may be necessary to vary the width of the cone and the quantity of light by use of the diaphragm which is always placed on all the better microscopes as nearly as possible even with trie top surface of the stage. "When no condenser is used the size of the opening in the diaphragm should be about that of the front lens of the objective. For some objects and some objectives this rule may be quite widely departed from; one must learn by trial."* The concave mirror acts as a lens and has a focus like a lens. It will often be found that by carefully focusing the mirror, details will be brought out clearly which otherwise would be but dimly seen. It may be found that in focusing up and down, the image shifts slightly from right to left, or to and fro. This may possibly be due to an imperfection in the microscope, but if the instrument is in good repair, and from any one of the reputable makers, the chances are more than likely that the shifting is due to oblique light, even though the mirror bar may be in the median line. This is even more apparent with a condenser than without it. Manufacturers are often condemned because of a mistaken idea that the mirror bar in this position means axial light. A slight turn of the mirror will stop the shifting and give axial illumination. When there is no lateral motion in focusing, the light is centered. Oblique Light. Some objects, such as diatoms, rulings, etc., are better defined when oblique light is used. This is accomplished without the condenser by swinging the mirror out of the optical axis and so turning it as to throw as much light as possible upon the object. When the ocular is removed the image of the mirror will be seen at one side o the center of the back lens of the objective. When focusing a decided lateral motion of the object will be noticed.
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Microscope Technique 
