508 DIOPTRIC IMPERFECTIONS. [BOOK HI. 



vertical plane of the line itself; and similarly, in order to see a 

 horizontal line distinctly it is much more important that the rays 

 whch diverge from the line in a series of vertical planes should be 

 brought to a focus properly than those which diverge in the 

 horizontal plane of the line itself. Hence a horizontal line held 

 before an astigmatic dioptric surface, most convex in the vertical 

 meridians, will give rise to the image of a horizontal line at the 

 nearer focus, the vertical rays diverging from the line being here 

 brought to a linear horizontal focus. Similarly, a vertical line 

 held before the same surface will give rise to an image of a vertical 

 line at the farther focus, the horizontal rays diverging from the 

 vertical line being here brought to a linear vertical focus. In 

 other words, with a dioptric surface most convex in the vertical 

 meridians, horizontal lines are brought to a focus sooner than are 

 vertical lines. 



Most eyes are thus more or less astigmatic, and generally with 

 a greater convexity along the vertical meridians. If a set of 

 horizontal or vertical lines be looked at, or if the near point of 

 accommodation be determined by Schemer's experiment (p. 494), 

 for the needle placed first horizontally and then vertically, the 

 horizontal lines or needle will be distinctly visible at a shorter 

 distance from the eye than the vertical lines or needle. Similarly, 

 the 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 Young. 



When the curvature of the comea or lens differs not in two 

 meridians only but in several, irregular astigmatism is the result. 

 A certain amount of irregular astigmatism exists in most lenses, 

 thus causing the image of a bright point, such as a star, to be not 

 a circle but a radiate figure. 



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 at some 

 distance the violet end will not be seen in focus at the same time 

 as the red. If a luminous point be looked at through a narrow 

 orifice covered by a piece of violet glass, which while shutting out 

 the yellow and green allows the red and blue rays to pass through, 

 there will be seen alternately an image having a blue centre with 



