LIGHT. 213 



meet in this focus. Those that are nearest the axis, R" F of the lens, 

 Fig. 82, are refract- 

 ed to a focus more Fi s- 8 ' 2 - 

 remote from the lens, 

 than those that fall 

 on the lens at a dis- 

 tance from the axis. 

 The rays R',R", and 

 E/", are brought to a 

 focus at F, whilst the 

 rays R L, and R"", 

 I/ converge at the 



point I, much nearer Aberration of Sphericity. 



the lens. In like 



manner, rays which fall upon the lens intermediate between the rays R 

 and R', will have their foci intermediate between I and F. This diver- 

 sity of focal distances is called spherical aberration or aberration of 

 sphericity: the distance I F is the longitudinal spherical aberration; 

 and B A the lateral spherical aberration, of the lens. This aberration 

 is the source of confusion in common lenses ; and as it is dependent 

 upon the shape of the lens, it has been obviated by forming these instru- 

 ments, of such degrees of curvature, that the rays, falling upon the 

 centre or margins of the lens, may be refracted to the same focus. 

 This is effectually accomplished by lenses, whose sections are ellipses 

 or hyperbolas. In a common lens, the inconvenience is obviated by 

 employing lenses of a small number of degrees, or by interposing an 

 opaque body called, by the opticians, a diaphragm anterior to the 

 lens, so that the rays of light can only impinge upon the central part, 

 and consequently be refracted to the same focus. This diaphragm is 



S:esent in all telescopes, and occupies the situation of the curves D and 

 ', so as only to admit the rays R', R", and R w , to fall upon the lens. 

 Such an apparatus, we shall find, exists in the human eye. 



Lastly, it has been already observed, that the different rays, con- 

 stituting the solar spectrum, are unequally refrangible, the red being 

 the least, the violet the most so; hence the cause of their dispersion in 

 the spectrum. It follows from this fact, that, whenever light expe- 

 riences refraction, there must be more or less dispersion of its consti- 

 tuent rays; and the object, seen by the refracted ray, will appear 

 coloured. This must, of course, occur more particularly near the 

 margins of the lens, where the surfaces become less and less parallel 

 until they meet. The inconvenience resulting from this dispersion is 

 called the aberration of refrangibility or chromatic aberration, and it 

 has been attempted to be obviated by glasses, which have been termed, 

 in consequence, achromatic. These are made by combining transparent 

 bodies of different dispersive powers, in such sort, that they may com- 

 pensate each other; and thus the object be seen in its proper colours, 

 notwithstanding the refraction. Dr. Blair found, for example, that 

 by enclosing chloride of antimony, B E, between two convex lenses of 

 crown glass, A D and C F, the parallel rays R R, and R were refracted 

 to a single focus at P without the slightest trace of secondary colour. 





