DESCRIPTION OF THE APPARATUS 



75 



mirror so set that it reflects the sunlight vertically 

 upwards through the tank. Thus illuminated, all 

 objects floating in the water appear bright, but the 

 water itself, if quite clear, is perfectly invisible. It 

 only now remains to form a dark background by 

 placing a black velvet curtain behind the transparent 

 portion, and to prevent the entrance of any outside 

 light. This is done by means of a light rectangular 



Fig. 50. — Dark background for the study of movements occurring in liquids. 



framework, pyramidal in form, and covered with a 

 black material. The base of the frame envelopes the 

 glass tank, and the other end receives the object-glass. 

 Ail opening made near the top allows the experimenter 

 to watch what is going on in the water, and to seize an 

 opportune moment for taking the photograph (Fig. 51). 

 Photography of Light Objects in Darkness or in a Red 

 Light. — Xo dark background is needed at night-time, 



