496 



NOTES. 



NOTE 192, p. 184. The object glass of the achromatic tele- 

 scope consists of a convex lens A B, fig. 55, of crown-glass placed 

 on the outside, towards the object, and of a concavo-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 pro- 

 portion as their dispersive powers, they will refract rays of light 

 without colour. 



NOTE 193, p. 188. When a sunbeam, after having passed through co- 

 loured glass V V, fig. 56, enters a dark room by two small slits Of in a 



Fig. 56. 



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 O or O/ 



