-405 



NOTE 193, p. 156. Fig. 53 represents the section of a poker, with 

 the refraction produced by the hot air surrounding it. 



NOTE 194, p. 156. 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 



Fig. 53. 



Fig. 54. 



f 



window-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 interposed, 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 coloured image V R of the sun, of the 

 same breadth with the diameter of the hole H, but much longer. The 

 space V R consists of seven colours violet, indigo, blue, green, yellow, 

 orange, and red. The violet and red, being the most and least refrangible 

 rays, are at the extremities, and the green occupy the middle part at G. 

 The angle D</G is called the mean deviation, and the spreading of the 

 coloured 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. 



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