216 PHOTO-MICROGRAPHY [CH VIII 



placed on the stage. For objective one of the objectives shown in Figs. ij6 to /So is 

 used. The objective is screwed into an adapter in place of the ordinary photographic 

 objective. The focusing is performed roughly by the rack and pinion, and then 

 with great exactness with the focusing glass. For manipulating the fine adjust- 

 ment of the special microscope the well known device of a cord over the head of the 

 micrometer screw is used. (See also Fig. 175.) ( Trans. Amer. Micr: Soc, 1001.) 



Successful photographs require an even lighting and an objective 

 which has sufficient field to take in the whole object. The camera 

 used for embryos (Fig. 175) answers very well for objects of moderate 

 size. For lighting them the specimen is put on the upper stage, and the 

 back-ground shown in the figure is removed. Then the large mirror 

 is used to throw light up through the preparation. If necessary the 

 specimen can be placed on one of the lower stories to bring it nearer 

 the mirror. The lighting and focusing should be as perfect as possi- 

 ble. Lamplight and daylight are both good. 



§ 353- Photographing Large Transparent Objects. — For this 

 the large vertical camera (Fig. 169, 181) is reversed in position on the 

 supporting frames, and elevated only sufficiently to make a sky back- 

 ground ; or a 45 degree reflector of white cloth or paper of sufficient 

 size must be used for a horizontal camera. If one has the earth for 

 back-ground the light will be dull and uneven and a very long expos- 

 ure is necessary, and the final results unsatisfactory. 



§ 354. Use of the Special Microscope Stand. — In order to 

 hold the specimen in position and to focus it accurately, it is put on 

 the stage of the special microscope stand (Fig. 175), which is inclined, 

 and fastened to the fixed part of the frame supporting the camera. As 

 the stage of this microscope is moved by the coarse or the fine adjust- 

 ment, the focusing can be accomplished with the same accuracy as the 

 microscope itself. For the general arrangement of the specimen and 

 the rough focusing the ground glass is used, then this is replaced by a 

 clear-glass focusing screen, and by the aid of a focusing glass the speci- 

 men is put in perfect focus. As one cannot reach the fine adjustment 

 while focusing, the well known device of a cord over the head of the 

 micrometer screw is resorted to. The two ends of the cord should be 

 weighted with about 50 or a hundred grams to keep the cord taut, then 

 whichever one is pulled, the micrometer screw will respond at once. 

 To cut off the light a piece of black velveteen is hung over the end of 

 the objective. This can be removed without jarring the apparatus. 

 An exposure of a few seconds (3 to 10 seconds), will suffice for many 

 preparations, unless a color screen is used. The color screen increases 

 the time of exposure from three to five times (§ 359). 



