THE IRIS. 



841 



Regular Astigmatism. When the curvature of the refracting surfaces of the 

 eye is unequally great in its different meridians, rays of light cannot be united 

 at a single point. Under such circumstances the cornea usually has the greatest 

 curvature in the vertical meridian and the smallest in the horizontal meridian, as 

 is shown by ophthalmometric measurement (p. 830). The rays that pass through 

 the vertical meridian naturally come together first, and in a horizontal focal line; 

 while the rays passing through the horizontal meridian are brought together further 

 back in a vertical line. Such an eye, therefore, does not possess a common focus for 

 light-rays: hence the name "astigmatism." The lens also exhibits some inequal- 

 ity of curvature in the various meridians, but just reversed. As a result, a part 

 of the inequality of curvature of the cornea is thus compensated, and only part 

 of it has any dioptric effect. The emmetropic eye possesses an exceedingly 

 slight degree of this inequality (normal astigmatism) . If two fine lines are drawn 

 at right angles to each other on a piece of white paper, it will be found that the 

 paper must be held closer to the eye, in order to see the 

 horizontal line distinctly, than to see the vertical line; the 

 normal eye is, thus, somewhat more short-sighted for 

 horizontal than for vertical objects. If the inequality of 

 curvature is more considerable, sharp vision naturally is 

 altogether impossible. For the correction of this error, a 

 glass is used that is ground in the form of a cylinder; that 

 is in one direction it has no curvature, while in the other 

 direction, perpendicular to the former, it is curved. The 

 glass is so placed before the eye that the direction of its 

 curvature corresponds to the direction of lesser curvature 



of the eye. For example, the section C a b c d of the 

 glass cylinder (Fig. 290) represents a plano-convex cylindri- 



a concavo-convex cylindrical 



FIG. 290. Cylindrical 

 Glasses for Astigma- 

 tism. 



cal glass; the section 

 glass. 



Irregular Astigmatism. As a result r of the stellate 

 arrangement of the fibers in the center of the crystalline 

 lens, and of the unequal course of the fibers within different 

 portions of one and the same lens-meridian, all of the 

 rays passing through one meridian cannot be focused at 

 the same point. For this reason sharp images of distant points of light (stars 

 or lamps) are not obtained, but rather stellate, jagged figures, with projecting 

 rays. The same thing may be seen by holding a card with a fine perforation 

 toward the light, at a somewhat greater distance from the eye than the far point. 

 Slight degrees of this irregular astigmatism are normal, but if developed to a high 

 degree the condition disturbs the visual acuity greatly, by producing several 

 images of each point of an object, instead of one image (monocular polyopia). 

 This condition cannot, of course, be present in eyes deprived of their lens. 

 Irregular curvatures of the cornea act in a similar way. A. E. Fick has elimin- 

 ated these by the use of a lens in the form of a watch glass, placed in contact 

 with the cornea (contact-spectacles); Lohnstein, by placing before the eye a 

 chamber closed in front by a spherical glass, and filling the interspace (between 

 the cornea and the spherical glass) with 0.85 per cent, solution of common salt 

 (hydrodiascope). 



THE IRIS. 



i . The iris acts like a diaphragm in an optical apparatus by cutting off 

 the marginal rays (Fig. 279), the entrance of which would produce a 

 decided spherical aberration, and as a result, indistinct vision. 2. As 

 the pupil contracts strongly in bright illumination, it regulates the 

 amount of light that enters the eye ; in this way fewer rays of light enter 

 the eye when the light is strong than when it is feeble. 3. The iris 

 acts, further, as an auxiliary to the muscle of accommodation. 



As the retina can adapt itself to a comparatively wide range of illumination, 

 the pupil (after the first reaction) can resume its size (from 3i to 4 mm.) if the 

 limits of illumination are between 100 and uoo meter-candles. 



