7 ^g ASTIGMATISM. 



Regular Astigmatism. When the curvature of the refractive surfaces of the eye is unequally 

 great in its different meridians, of course the rays of light cannot be united or focussed m one 

 point Generally, in such cases, the cornea is more curved in its vertical meridian and least m 

 the horizontal (as is shown by ophthalmometry? measurements, P- 748). The rays passing 

 through the vertical meridian come to a focus, first, in a horizontal focal line ; while the rays 

 entering horizontally unite afterwards in a vertical line. There is thus no common focus lor 

 the light ravs in the eve ; hence the name "astigmatism." The lens also is unequally curved 

 in its meridians, but it is the reverse of the cornea ; consequently, a part of the inequality of 

 the curvature of the cornea is thereby compensated, and only a part of it affects the rays of 

 light The emmetropic eye has a very slight degree of this inequality (normal astigmatism). 

 If two very fine lines of equal thickness be drawn on white paper, so as to intersect each other 

 at right angles, it will be found that, in order to see the horizontal line quite sharply, the paper 

 must W brought slightly nearer to the eye, than when we focus the vertical line. \N hen the 

 inequality of curvature' of the meridians is considerable, of course exact vision is no longer 

 i>ossible. , . .. , _ , . . , 



[Fig. 542 shows the effect of an astigmatic surface on the rays of light. Let a b c d be such 

 a surface, and suppose diverging rays to proceed from/. The rays passing through c d come to 



Fig. 542. 

 Action of an astigmatic surface on a cone of light (Frost). 



a focus at/,, while those passing through the vertical meridian are focussed at/* The outline 

 of the cone of rays between abed and/ 2 varies, as shown in the figure. At a certain part it 

 is oval, with its axis vertical, at another "the long axis of the oval is horizontal, while at other 

 places it is circular, or the rays are focussed in a horizontal or vertical line.] 



Correction. This condition is corrected by a cylindrical lens, i.e., a lens so cut as to be 



without curvature in one direction, while in the other direction (vertical to the former) it is 



curved. The lens is placed in front of the eye, so that the direction of its curvature coincides 



-, - with the direction of least curvature of the eye (v. Helmholtz, Knapp, Danders). 



^-yr J* The section C a b e d of the cylindrical lens (fig. 543) represents a plano-con- 



( f\ \)\ vex, the section C a y 5, a concavo-convex lens. 



[Test. Draw two lines of equal thickness at right angles to each other. An 

 astigmatic person cannot see both lines with equal distinctness at the same 

 time, one line will appear thicker than the other. Or, a series of lines radiating 

 from a centre may be used (astigmatic clock) when that line which is parallel 

 to the astigmatic meridian will be seen most distinctly ; while, with the ver- 

 tical meridian most curved, it would be the vertical line.] 



Irregular Astigmatism. Owing to the radiate arrangement of the fibres in 

 the interior of the crystalline lens, and in consequence of the unequal course 

 of the fibres within the different parts of one and the same meridian of the lens, 

 the rays of light passing through one meridian of the lens, cannot all be 

 brought to one focus. Hence, we do not obtain a distinct sharp image of 

 distant luminous points, such as stars or street lamps, but we see a radiate 

 jagged figure provided with rays. The same obtains on holding a piece of card- 

 board with a small hole in it towards the light, at a distance from the eye 

 slightly greater than the far point. Slight degrees of this irregular astigma- 

 tism are normal, but when it is highly developed it greatly interferes with 

 vision, by forming several foci of an object instead of one (Polyopia monocu- 

 laris). Of course this condition cannot obtain in an eye devoid of a lens. 



392. IRIS. Functions. 1. The iris acts like a diaphragm in an optical 

 apparatus by cutting off the marginal rays, which, if they entered the eye, would 

 cause spfterical aberration, and thus produce indistinct vision (fig. 531). 



2. As the pupil contracts strongly in a bright light, and dilates when the light 

 is feeble, it regulates the amount of light entering the eye ; thus, fewer rays enter 

 the eye when the light is strong than when it is feeble. 



Fig. 543. 



Cylindrical 



glasses for a 



stigmatism. 



