572 SIGHT. 



image when the edges of bodies are brought near the side of the 

 fasciculus of rays which enter the eye : prismatic colours also 

 appear. All these phenomena, which are observed in an eye 

 performing its functions, may be produced by an apparatus, the 

 structure of which resembles that of the eye ; and even by a com- 

 mon lens, if we substitute for the motion of the pupil diaphragms 

 of different sizes. The nature of all these phenomena proves that 

 diffraction is their common origin, and they may be considered as 

 constituting a separate kind of optical illusions resulting from 

 diffraction. The second cause which adapts the eye for the mo- 

 mentaneous distinct vision of objects depends neither on the 

 action of the external muscles of the eye, the advancement of the 

 bottom of the eye, nor on any alteration of the form or position 

 of the crystalline lens ; but appears to be owing rather to the 

 change of the curvature of the cornea by the contraction of the 

 iris, which occurs only when the eye adapts itself to see very 

 near objects, as is proved by the simultaneous approximation of 

 the pupil. 



Besides the motions of the pupil which accompany the adapt- 

 ation of the eye to different distances, it also varies with the in- 

 tensity of the light to which the eye is exposed, contracting in a 

 strong light, and dilating in a faint light. This effect is particularly 

 observable in persons whose pupils are large. The sympathetic 

 action of the pupils of both eyes is worthy of remark : while the 

 same light is regarded, the pupil of one eye will dilate imme- 

 diately the other eye is covered, and again contract when the 

 hand is removed and both organs are equally exposed to the 

 light. 



Correction of spherical aberration in the eye. In ordinary lenses 

 the rays which pass through them at different distances from the 

 centre are not refracted to the same point or focus, and the images 

 they form are consequently indistinct. This defect, which is 

 called spherical aberration, is, according to Sir D. Brewster, re- 

 medied in the eye by means of the variable density of the crys- 

 talline lens. The refractive power of this body being greatest at 

 its centre, and decreasing towards its circumference, the central 

 and extreme rays are both refracted to the same point. Ac- 

 cording to the experiments of M. Chossat, the ellipsoidal figure 

 of the cornea is sufficient to effect this correction. 



Chromatic aberration. In consequence of each differently 



