466 PHYSICAL SCIENCES. 



NOTE 195, pp. 159, 164. Under the same circumstances, and where the 

 refracting angles of the two prisms are equal, the angles D</G and V</R, 

 tig. 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 crow r n-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 conclusion 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. 



NOTE 196, p. 165. The object glass of the achromatic telescope consists 

 Fiq. 55. of a convex leus A B ' % 55 > of crown-glass placed on the 

 outside, towards the object, and of a concave-convex lens C D 

 of flint-glass, placed towards the eye. The focal length of 

 a lens is the distance of its centre from the point in which 

 the rays converge, as F, fig. 60. If, then, the lenses A B 

 and C D be so constructed that their focal lengths are in the 

 same proportion as their dispersive powers, they will refract 

 rays of light without colour. 



NOTE 197, p. 165. If the mean refracting angle of the 

 prism D#G, fig. 54, were the same for all substances, then 

 the difference Dg V - Dg R would be the dispersion. But the 

 angle of the prism being the same, all these angles are dif- 

 ferent in each substance, so that in order to obtain the dis- 

 persion of any substance the angle DgV DgR must be 

 _ divided by the angle Dg G or its excess above unity, to which 

 the mean refraction is always proportional. According to 

 Mr. Fraunhofer the refraction of the extreme violet and red rays in crown- 

 glass is 1-5466 and 1-5258; so D#V-D# R= 1-5466- 1-5258 = '0208, 

 and half the sum of the excess of each above unity is -5362 ; consequently 



-0208 



=0-03879 ; for diamond 



so that the dispersive power of diamond is a little less than that of crown- 

 glass ; hence the splendid refracted colours which distinguish diamond 

 from every other precious stone are not owing to its high dispersive power, 

 but to its great mean refraction. SIR DAVID BREWSTER. 



NOTE 198, p. 168. When a sunbeam, after having passed through a 

 coloured glass V V, fig. 56,- enters a dark room by two small slits OO' in 

 a card, or piece of tin, they produce alternate bright and black bands on a 

 screen S S' at a little distance. When either one or other of the slits 

 or 0' is stopped, the dark bands vanish, and the screen is illuminated by a 

 uniform light, proving that the dark bands are produced by the interference 

 of the two sets of rays. Again, let H m, fig. 57, be a beam of white light 



