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Microscope Illumination with a Substage Condenser.
Microscope Illumination with Substage Condenser
Central Light. All of the better microscopes are provided with a condenser fitted beneath the stage, which brings parallel rays of light to a focus at a point above its upper surface. With the lowest powers a condenser is not needed, but for the medium and higher powers the condenser not only furnishes the amount of light needed, but provides an easy means of providing each objective with a cone of light suitable to its aperture. Condensers are of two great classes the archomatic and non-achromatic. The achromatic are much the better, and are indispensable for photo-micrographic work, but as the non-achromatic (Abbe) is in such general use on account of its lesser cost and because it is sufficient for ordinary work, we will consider it especially. The same general rules apply to both, which are fundamentally the same as those given before, with the exception that in most cases the plane mirror is used because, as stated above, the condenser is made for parallel rays of light. After removing the ocular, turn the mirror so that the back lens of the objective is fully and evenly illuminated and, if possible, free from any images of trees, window sash, etc. If these images cannot be dispelled by turning the mirror, use the concave mirror. Slightly lowering the condenser will also accomplish the end. There is an objection to both of these methods, which will be explained later. When this is accomplished the proper cone of light must be secured by opening or closing the diaphragm below the condenser. A good general rule is to close the diaphragm so that in looking at the back lens of the objective the diaphragm opening, which can be plainly seen, appears to be about half the diameter of the back lens of the objective when it is in focus. Then with the ocular in place change the opening to give the best results. The thinner the tissues and the greater the contrasts the larger the cone of light which may be used. Thicker tissues and those with less 19 contrast require a narrower cone, gaining thereby greater depth of sharpness (penetration). The narrower the cone the flatter the field appears. Very few objects permit of a cone which fills the back lens of the objective and in no case should the diameter of the iris diaphragm appear to be larger than the diameter of the back lens when the ocular is removed. When objectives of over i.o N. A. (immersion objectives) are used, the full aperture of the condenser cannot be utilized without immersing it, i. e., placing a drop of oil between it and the lower surface of the slide. This is seldom practiced in general laboratory work on account of its inconvenience, but it is necessary to the most critical work. In working with artificial light it is always best to use a bull's eye condenser. If no bull's eye is available, use the concave mirror and so turn it that the image of the light source appears in the centre of the back lens of the objective when the ocular is removed. When the bull's eye is used, select the plane mirror and so place the light source and bull's eye that the image of the light source appears natural size on a cardboard placed at the back of the condenser. Remove the bull's eye and put the light source in its place. Focus the substage condenser so that the image of the light source appears in the plane of the object. This is best seen by using a low power objective and ocular. Now put the bull's eye in its former position after removing the light to its original place, or as before, so arrange the light source that a sharp image of the light source appears on the back of the condenser iris, or a card placed against it. This will give an even illumination. A blue glass should also be used beneath the condenser, unless the blue globe mentioned is used. This modifies the yellow artificial light. Oblique Light. Even though both the condenser and iris are' centered, central or axial light is not obtained, unless the rays of light, or the axis of the cone of light, from the mirror enter the condenser parallel with its axis. This fact is often forgotten as was pointed out before. Beside the turning of the mirror oblique light can be obtained in a greater degree by decentering the lower iris and in the best microscopes the obliquity can be obtained from any azimuth'by revolving the diaphragm mounting. With the simple diaphragm mounting with a ring beneath the diaphragm for blue glass oblique light can be obtained by slipping a card between the ring and diaphragm in such a manner as to let the light into one side of the condenser only.
Illumina Tionopa Que There are some objects which cannot be made transparent and must be examined by reflected light. When low powers are used and the mirror brought above the stage the concave mirror is sometimes sufficient. The bull's eye condenser gives better results. In using it some care must be exercised to light the object and at the same time cast as few shadows as possible. Where high powers with consequent short working distances are necessary, light must be thrown down through the objective and reflected back again to the eye by a prism placed in the objective mount or above it. Artificial light is necessary with such an arrangement. Focusing the Condenser Nearly every substage is provided with a means for focusing the condenser. The condenser does its best work only when the rays of light passing through it are focused sharply on the object. If there are any reflections from the mirror of trees, window sash, etc , they will show on the back lens of the objective when it is focused and when the condenser is in this position. The operator must decide for himself as to whether he gets better results with the sharp focus and the images, or without either. If he decides to retain the images he can get best results by turning the mirror so that they are as symmetrically distributed over the back lens as possible. This emphasizes the importance of a clear, open source of light. Centering the Condenser For central light the axis of the condenser should coincide with the axis of the objective and the center of the opening of the diaphragm beneath the condenser should also be in this axis. On most of the medium grade microscopes the iris is so fastened to the condenser that it is concentric with its axis, and both are generally centered with the optical axis of the body tube before leaving the factory. This can be tested by a simple means. Close the diaphragm to its smallest aperture and notice this aperture through the back lens of the objective. If the condenser is centered this opening will appear in the center of the lens and will remain concentric with the periphery of the lens when the objective is focused up and down. The periphery of the top surface of the condenser will also be concentric with the periphery of the back lense of the objective. On the more expensive microscopes there are little screws provided for moving the condenser in and out of center. By means of these the condenser can be centered by observing the above rule. On these instruments the iris diaphragm is brought in and out of center by a rack and pinion, a click indicating when it is centered.
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Microscope Technique 
