198 



EXPERIMENTS MADE IN VIRGINIA. [MEMOIR XIII. 



Fig. 25. 



heat -rays could be ascertained by an air- thermometer, 

 and the acting chemical rays by papers made sensitive 

 by chloride of silver. 



Into a darkened chamber, the shutter of which is rep- 

 resented in section at A A, Fig. 25, a beam of the sun's 



light is made to pass hori- 

 zontally by means of a mir- 

 ror of silvered glass, B. The 

 mirror I use is one belong- 

 ing to a solar microscope, 

 and by turning the milled 

 screws, e e, it can be brought 

 into any position required 

 to throw a beam horizontally into the room. A brass 

 tube,/, two inches in diameter, can be screwed into the 

 position figured. There is also a lens, a, Fig. 31, which 

 may be introduced ; its focus is nine inches, its diameter 

 about two inches, and the diameter of the sun's image it 

 gives nearly -^ of an inch. 



A piece of sheet-lead about a quarter of an inch thick 

 is to be cut into the form of a horse-shoe of such size 

 that a circle one inch in diameter may be inscribed in it. 

 Upon this lead two pieces of glass are cemented, so as to 

 form a trough for containing liquids. In Fig. 26, a a is 

 the wooden basis or foot of the trough ; 

 c c c, the leaden horse -shoe; b b, the 

 glass plates. 



A thin metallic plate three or four 

 inches square, having a longitudinal slit 

 in it about an inch long 

 and ^ f an i nc h wide, is 

 to be provided. In Fig. 27, a a is the slit. 

 The lens, $, Fig. 31, having been removed, 

 a beam of light is thrown horizontally into 

 the room, as at Fig. 25 ; the slit, Fig. 27, 



Fig. 26. 



Fig. 27. 



