USE AND MANIPULATION OF MICEOSCOPE 147 



through the eye-piece of the microscope an evenly illuminated 

 field results. 



In any case, before taking a photograph this operation with 

 greater or less elaboration will be necessary, so that the part of 

 the object required in the field of view may be brought into 

 position. 



In observing the image, if the worker has not already accus- 

 tomed himself to do so, he should at once endeavour to train 

 himself so that both eyes always remain open. This applies 

 not only when observing the image visually, but also when 

 focussing, the image being projected on the ground-glass screen 

 of the camera and observed with a focussing ocular. At 

 first this may seem a trifle difficult, as extraneous objects become 

 painfully evident. But it will be found with some little 

 practice, especially if the eye not in use is for a time partially 

 shaded with the hand or with a piece of black cardboard, that it 

 soon becomes an easy matter to leave both eyes open. Both 

 eyes should be used alternately, so that there is no greater strain 

 on one than the other. Where long searches have to be con- 

 ducted it is a great relief to change from one eye to the other 

 at short intervals, thus avoiding the risk of any undue strain 

 and consequent fatigue. 



The object having been set up, the most instructive pro- 

 cedure is to turn the microscope into the horizontal position and, 

 instead of viewing the image directly, to project it on to a small 

 white screen, such as a piece of white cardboard some eight or 

 ten inches square, placed, say, at a distance of eighteen or twenty 

 inches from the ocular. The mirror must of course be either 

 swung aside or removed. It is with projected images that in 

 photo-micrography the worker is almost entirely concerned, and 

 some preliminary experiments in simple projection will be far 

 more useful than to begin at once exposing plates. 



When the microscope is in the horizontal position, assuming 

 that the object has been approximately focussed and the 

 source of light placed so that it is properly illuminated, 

 the projected image will be seen on the white screen placed as 

 suggested. It may be found that in any case some alteration 

 in the position of the illuminant will be necessary before the 

 field becomes evenly illuminated. It will somewhat facilitate 

 perfect centration if the iris-diaphragm be now closed, so that 



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