328 



WELLS'S NATURAL PHILOSOPHY. 



red, which passes upward into orange, then into yellow, then green, blue, 

 indigo, and violet, which is at the upper end. 



Dissimilar substances, however, produce spectra of dilTerent lengths, on ac- 

 count of a diflFerence in their refractive properties. Thus a ray of light tra- 

 versing a prism of llint-glass, will have its red aud violet colors separated on 

 a screen twice as widely as those of a ray passing through a similar prism 

 of crown-glass. This diflcrence is expressed by saying that the dispersive 

 power of the two substances is different, or that flint-glass has twice the dis- 

 persive power of crown-glass. 



-„ .„ , As a lens may be considered as a modification of the 



WTiy -will not •' 



»n ordinary prism, it follows that when light is refracted through a lens, 



lens produce a jj jg separated into the diQ'erent colors, precisely as by a 

 perfect image? '■ ^ t- j j 



prism ; and as every ray contained in white light is refracted 



differently, every lens, of whatever substance made, will have a different focus 

 for every different color. The images, therefore, of such lenses will be more 

 or less indistinct, and bordered with colored edges. This imperfection is 

 termed chromatic aberration. 



For this reason the focus of a burning-glass, which is an optical image of 

 the sun, is never perfectly distinct, but always confused by a red, or blue bor- 

 der, since the various-colored rays of which sunlight is composed, can not 

 all be brought to the same focus at once. In a like manner, if we point a 

 common telescope at a blue and red hand-bill at a short distance, we shall 

 have to draw out the tube of the instrument to a greater length in order to 

 read the red than the blue letters. 



These fringes of color are a most serious obstacle to the 

 Kxplam the . . X^ . , . .... . , 



construction of perfection of optical instruments, especially in astronomical 



an achromatic telescopes, where great nicety of observation is required ; and 

 to prepare a lens in such a way that it would . fract light 

 ■nnthout at the same time dispersing it into colors, was long consider ;1 an im- 

 possibility. 



The discovery was, however, made by Mr. Dollond, 

 an Englishman, that by combining two lenses, formed 

 of materials which refract light differently, the one 

 might be made to counteract the effects of the other; on 

 the same principle as by combining two metals together 

 which expand unequally, we may construct a pendu- 

 lum whose length never varies. 



Such a combination is represented in Fig. 268, whero 

 a convex lens of crown glass is united with a concave 

 lens of flint glass, so as to destroy each the dispersivo 

 power of the other, while at the same time the refract- 

 ing, or converging power of the convex lens is pre- 

 served. A lens of this character is called Achro- 

 matic,* since it produces images in their natural 

 colors. 

 * Achromatic, from a, not, and xp<^f^<^i color. 



Fig. 268. 



