50 ELEMENTARY PHOTO-MICROGRAPHY. 



be fed through a tube, though the smoke from this 

 is objectionable in ordinary households, where it 

 is presumed the reader will usually work. 



If care be taken to get uniform illumination by 

 means of ground-glass, then much time is saved by 

 this light, which is rich in actinic rays, but the be- 

 ginner is not advised to try it until he has a good 

 knowledge of all the points to be observed in the 

 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. 



