The Achromatic Telescope. 455 



which, is known by the name of "spherical aberration," or 

 sometimes " aberration" alone, may also be illustrated by the 

 diagram at p. 215 of our number for April, 1863, where a corre- 

 sponding, though not precisely similar, defect is shown to be 

 the result of employing a spherical surface in reflection. 



The second source of error is of an entirely independent 

 nature, arising from the unequal refrangibility of the rays of 

 different kinds of light. If all the coloured rays which by their 

 union compose whiteness, were acted upon to the same extent 

 by refraction, they would still remain united under whatever 

 circumstances refraction might occur, and no decomposition 

 into colour could ever take place by any process of this kind. 

 But such is not the case. Red light is invariably less bent 

 from its course than yellow, yellow less than blue ; and, there- 

 fore, refraction never takes place, even in the smallest degree, 

 without a corresponding separation of the light into the primary 

 colours of red, yellow, and blue, which by overlapping and 

 partial combination produce the seven colours of the rainbow. 

 The focus of the red light will for this reason be furthest from 

 the lens, that of the yellow intermediate, and that of the blue 

 shortest, the intervening spaces being filled up by various com- 

 binations of colour ;* and whichever focus we may assume, the 

 image there produced will not merely be deficient in due bright- 

 ness, being formed by a portion only of the rays proceeding 

 from the object, but will be bordered by fringes formed by the 

 other colours converging to their own independent foci. The 

 general cone of rays, as intercepted on a paper screen, will 

 appear white, for though the whole mass of light is actually de- 

 compounded, an infinite number of cones of different colours 

 being really produced by the infinite number of concentric 

 rings of which the lens may be supposed to consist, yet as these 

 all overlap and intermingle with each other, the whiteness is 

 recomposed throughout. The outermost colour, however, hav- 

 ing nothing to overlie it, crops out, as it were, in the form of a 

 tinted border, and thus the general cone within the focus is 

 fringed with the less rapidly convergent red, and beyond the 

 focus by the more refracted blue, which having converged to a 

 point nearer to the lens, but invisibly so, being neutralized in 

 the general cone, crosses the general mass (confusing the other 

 foci in its passage), and becomes outermost in the subsequent 

 divergence. 



This may be exemplified in an interesting manner by stick- 



* The innermost focus is actually, as it is generally stated to be, that of the 

 violet light ; but we have taken no notice of it in the present explanation, partly 

 because, according to Sir D. Brewster, it is not a primary colour, but compounded 

 of the extreme red and blue rays, and partly because it is not of sufficient intensity 

 to be recognized in these experiments, 



