758 



SIGHT. 



erly than those which diverge in the vertical plane of the line itself; and 

 similarly, in order to see a horizontal line distinctly, it is much more im- 

 portant that the rays which 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 further 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. 747), 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 further 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 some- 

 times the fault lies in the lens, as was the case with Young. 



When the curvature of the cornea 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. 



643. Chromatic aberration. The different rays of the spectrum are 

 of different refrangibility, those toward 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 



Diagram illustrating Chromatic Aberration, h h is the dioptric surface ; h v represents the blue 

 and h r the red rays ; V is the focal plane of the blue, R of the red rays. 



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 a red fringe, or a red centre with a blue fringe, according as the 

 image of the point looked at is thrown on one side or other of the true 

 focus. Thus supposing /(Fig. 173) to be the plane of the mean focus of 

 A, the violet rays will be brought to a focus in the plane V, and the red 



