ELEMENTARY PHOTO-MICROGRAPHY. 



note the plate used, the class of object photo- 

 graphed, with its characteristic quality, the ex- 

 posure, the distance of the plate from the object, 

 and the result with each power and with each 

 class of illumination. Supposing this to be done, 

 and one wishes to know what exposure to give to 

 a botanical section stained red, using a 2in. objec- 

 tive and the A eyepiece, reference to the register 

 shows, say, the following : 



TRANSMITTED LIGHT. 

 ain objective. " A " eyepiece. 



This will give an idea of the correct exposure 

 for any similar class of specimen. It is folly to 

 rely on the memory alone, like so many landscape 

 photographers do. Far better take every precau- 

 tion suggested by previous experiments to ensure 

 correct exposure. One must not move about dur- 

 ing the time of exposure, for vibration will spoil 

 the sharpness of any negative, especially with high 

 powers. It is not necessary, however, to remain 

 inactive, for the gas may be turned full on, and 

 the time occupied by reading or writing. When 

 very long exposures are made, the worker may 

 even leave the room and return at the end of an 



