50 



CHROMATIC ABERRATION. 



[BOOL III. 



distance from the eye at which the horizontal lines or needle are 

 seen distinctly will be found, in most cases, to be appreciably 

 and in many cases considerably shorter than that at which the 

 vertical lines or needle are seen with equal distinctness. In other 

 words, in the case of most eyes, a vertical line must be farther 

 from the eye than a horizontal one, if both are to be seen 

 distinctly at the same time. The cause of astigmatism is, in 

 the great majority of cases, the unequal curvature of the cornea ; 

 but sometimes the fault lies in the lens, as was the case with the 

 philosopher Young. 



Regular astigmatism may be remedied by the use of cylindrical 

 glasses, that is to say, glasses which are convex along one meridian 

 but plane along the other. Thus the ordinary astigmatic eye 

 with the greater curvature along the vertical meridian will be 

 benefited by a cylindrical glass, plane in the vertical plane but 

 possessing such convexity in the horizontal plane as will make up 

 tor the relatively deficient horizontal curvature of the cornea. 



When the curvature of the cornea or of the lens differs not in 

 two meridians only but in several, irregular astigmatism is the 

 result. A certain amount of irregular astigmatism, due to the 

 cornea or lens, exists in most eyes, thus causing the image of a 

 bright point, such as a star, to be not a round dot but a radiate 

 figure ; in some cases the irregularity is so great that several 

 imperfect images are formed of every object. 



735. Chromatic aberration. The different rays of the 

 spectrum are of different refrangibility, those towards the violet 

 end of the spectrum being brought to a focus sooner than those 

 near the red end. This in optical instruments is obviated by 

 using compound lenses made up of various kinds of glass. In the 

 eye we have no evidence that the lens is so constituted as to 

 correct this fault ; still the total dispersive power of the instrument 

 is so small, that such amount of chromatic aberration as does exist 

 attracts little notice. Nevertheless some slight aberration may be 

 detected by careful observation. When the spectrum is observed 



FIG. 143. DIAGRAM ILLUSTRATING CHROMATIC ABERRATION. 



hh is the dioptric surface, hv represents the blue, and hr the red rays; Vis the focal 

 plane of the blue, R of the red rays. 



at some distance off the violet end will not be seen in focus at the 

 same time as the red end. Again, if a luminous point be looked at 

 through a narrow orifice covered by a piece of violet glass, which 



