4 o 



ELEMENTARY PHOTO-MICROGRAPHY. 



what exposure to give to a botanical section stained 

 red, using a 2in. objective and the A eyepiece, refer- 

 ence to the register shows, say, the following : 



TRANSMITTED LIGHT. 

 2in. 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 photo- 

 graphers do. Far better take every precaution sug- 

 gested bv previous experiments, to ensure correct 

 exposure. One must not move about during the 

 time of exposure, for vibration will spoil the sharp- 

 ness 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 photographing assorted 

 specimens. The distance of the object from the 

 plate was about eighteen inches, the source of 

 illumination a paraffin lamp with a one-inch wick. 



