424 



NOTES. 



Fig. 53. 



NOTE 189, p. 151. Fig. 53 represents the / ; 

 section of a poker, with the refraction pro- ; / 

 duced by the hot air surrounding it. 



NOTE 190, p. 153. The solar spectrum. A ray from the sun at S, fig. 

 54, admitted into a dark room through a small round hole H in a vvindow- 



TOxkte 



shutter, proceeds in a straight line to a screen D, on which it forms a 

 bright circular spot of white light of nearly the same diameter with the 

 hole H. But when the refracting angle B A C of a glass prism is inter- 

 posed, so that the sunbeam falls on A C the first surface of the prism, and 

 emerges from the second surface A B at equal angles, it causes the rays 

 to deviate from the straight path S D, and bends them to the screen M N, 

 where they form a colored image VR of the sun, of the same breadth 

 with the diameter of the hole H, but much longer. The space V R con- 

 sists of seven colors, violet, indigo, blue, green, yellow, orange, and red. 

 The violet and red, being the most and less refrangible rays, are at the 

 extremities, and the green occupy the middle part at G. The angle D g G 

 is called the mean deviation, and the spreading of the colored rays over 

 the angle V g R the dispersion. The deviation and dispersion vary with 

 the refracting angle B A C of the prism, and with the substance of which 

 it is made. 



NOTE 191, p. 159. Under the same circumstances, and where the re- 

 fracting angles of the two prisms are equal, the angles D^G and \ g R, 

 fig. 54, are greater for flint-glass than for crown-glass. But as they vary 

 with the angle of the prism, it is only necessary to augment the refracting 

 angle of the crown-glass prism by a certain quantity, to produce nearly 

 the same deviation and dispersion with the flint-glass prism. Hence, 



