PHOTOMICROGRAPHY. 



139 



portion of a considerable image of light. My custom is to nearly close the iris 

 diaphragm below the Abbe condenser and throw with the condensing lens a small 

 circle of light into the center of this diaphragm ; the condensing lens is then slid 

 along the track about 12 or 15 inches nearer; the iris diaphragm is then opened 

 wide and the exposure made at once by squeezing the bulb of the shutter. 



I now always use apochromatic lenses and never make negatives without an 

 eye-piece. I have used Zeiss projection oculars, but now use in preference a Zeiss 



Fig. 127.* 



No. 4 compensating ocular, or Spencer No. 3, which is kept solely for this purpose 

 (so as to be always clean). It is of the utmost importance that mirror, walls of 

 light-filter, alum-cell, and surfaces of condenser, slide, objective, and ocular be abso- 

 lutely free from dirt, grease, and dust particles, even the smallest, if a good negative 



*Fic. 127. Bacterial leaf-spot of the larkspur (Delphinium). Same as fig. 126, but photo- 

 graphed on Cramer's isochromatic slow plate. In this photograph the black spots on a green back- 

 ground come out distinctly; in fig. 126 they do not 



