Chap, hi.] 



SIGHT. 



873 



generally with a greater convexity along the vertical meridian. 

 If a set of horizontal or vertical lines be looked at, or if the 

 near point of accommodation be determined by Schemer's exper- 

 iment, for the needle placed first horizontally and then verti- 

 cally, the 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 ver- 

 tical line must be farther from the ej^e 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 cylin- 

 drical 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 for 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. 



§ 548. 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 



Fig. 148. Diagram illustrating Chromatic Aberration. 



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

 plane of the blue, B of the red ray. 



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 



