46 ELEMENTARY PHOTO-MICROGRAPHY. 



taking of a perfect negative. When using very 

 oblique illumination, an exposure of two or three 

 minutes with magnesium will be found equal to 

 sixty minutes with paraffin lamp, and with high 

 powers the shorter the exposure the better. 

 With very long exposures through an oil-immersion 

 objective there is a danger that the focus will be 

 altered by a variation of the temperature of the 

 room, for even a change of a few degrees may be 

 fatal to the stability of the image. 



