NOTES. 



495 



NOTE 190, p. 178. 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 window-shutter, 



Fig. 54. 



proceeds n a straight lint to a screen D, on which it orms 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 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. 



NOTE 191, p. 184. Under the same circumstances, and where the refracting 

 angles of the two prisms are equal, the angles DgG and V#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, when the two prisms are placed 

 with their refracting angles in opposite directions, as in fig. 54, they nearly 

 neutralize each other's effects, and refract a beam of light without resolving it 

 into its elementary coloured rays. Sir David Brewster has come to the con- 

 clusion that there may be refraction without colour by means of two prisms, 

 or two lenses, when properly adjusted, even though they be made of the same 

 kind of glass. 



