148 POPULAR SCIENCE MONTHLY. 



slides that may be readily photographed with the microscope in a 

 horizontal position. A mechanical stage is of great convenience in this 

 work as it saves much time in accurately placing a slide, especially 

 where high power objectives are used and when focusing must be done 

 at arm's length. In the making of photographs of rock sections, it is 

 frequently necessary to use Nicol prisms in order to properly differen- 

 tiate particular portions. The one is placed over the ocular, or be- 

 tween it and the objective, and the other beneath the sub-stage con- 

 denser. As the latter is free to be rotated, it is an easy matter to 

 bring out clearly the special features desired. Fig. 2 illustrates 

 the apparatus in a vertical position. It must be operated in this 

 position when freshly mounted slides are used and where photo- 

 micrographs are to be made of living organisms. A special shutter 

 devised by the writer for the photographing of living specimens, 

 a detailed description of which will be found in the Scientific American 

 of March 24, 1900, is shown in place on tbe draw tube of the microscope. 

 In place of the mirror a hole is drilled through the base of the camera 

 stand in order to make transmitted light available. With this arrange- 

 ment, as illustrated, photographs have been made in 1/50 of a second, 

 (see Figs. 3 and 4). 



"1 



A 



Fig. 3. Phyllopod. Line Specimen. Fig. 4. Daphina. Line Specimen. X150. 



X 250. Exposure & Sec. Exposure e \, sec. 



The character of the plates used in connection with the work and 

 a word as to their manipulation is worthy of notice, because ultimate 

 results are not dependent upon first quality optical apparatus alone. 

 With ordinary light and low power objectives, a slow plate is to be 

 preferred, and with such conditions the Carbutt's B brand has given 

 excellent results. Where polarized light is employed a color sensitive 

 plate is preferable. Such light often gives the most beautiful colors 

 with rock sections and crystal specimens, and the Cramer isochromatic 

 plate renders the color values admirably. 



The question of exposure must be decided principally by experi- 

 ence. The candle power of the illuminant; the character of the 



