THE PRISMATIC SPECTRUM. 



shown in fig. 6, be placed at A B c, with the edge of its refracting 

 angle B in a horizontal direction, and presented downwards so as 



Fig. 5. 



Fig. 6. 



to receive the pencil upon its side A B at Q. According to a 

 well known principle in optics, the pencil would bs refracted, in 

 passing through the surface A B, in the direc- 

 tion Q, L towards the perpendicular ; and it 

 would be again refracted, in emerging from 

 the surface c B, from the perpendicular in the 

 direction L K. It might therefore be expected 

 that the effect of the prism would be merely 

 to move the spot of light from p' to some point, 

 such as K, more elevated upon the screen. 

 The phenomenon, however, will be very diffe- 

 rent. Instead of a spot of light, the screen 

 will present an oblong coloured space, the 

 outline of which is represented at M N as it 

 would appear when viewed in front of the 

 screen. A perspective view of the arrange- 

 ment for making this celebrated experiment 

 is given in fig. 7, p. 65. 



17. The sides of this oblong figure are parallel, straight, and 

 vertical. Its ends are semi-circular, and its length consists of 



r2 67 



