172 



PRACTICAL PHOTOMICROGRAPHY. 



the exposure will be protracted. During the long exposure 

 there is great risk of motion of the apparatus not only from 

 tremor of the apartment, but from heat of the radiant affecting 

 the apparatus. An alum trough is therefore recommended be- 

 tween light and condenser. 



FIG. 37. 



If the edge of a lamp-flame or the face of a lime cylinder is 

 used as the illuminating surface, the frustule must be arranged 

 vertically on the stage, and the slot of the condenser stop is 

 to run parallel to the frustule. The directions of valve and 

 light are therefore shown thus : Fig. 38, while in the field of 



FIG. 



the microscope almost the whole will be dark, except the 

 frustule itself, until the high power o. g. is brought into play. 



