86 ELEMENTARY PHOTO-MICROGRAPHY. 



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 

 appointed time, so being free to do other things 

 meanwhile. 



The conditions governing correct exposure are 

 so varied that no rule can be stated, and the 

 following examples are merely given to point out 

 the probable variations likely to occur in photo- 

 graphing assorted specimens. The distance of the 

 object from the plate was about i8in., the source 

 of illumination a paraffin lamp with a lin. wick. 



