794 SPECIAL SENSES. 



in normal vision, rays from the same object cannot fall upon both points at the same time. 

 Thus, in binocular vision, the insensibility of the punctum caecum does not interfere with 

 sight ; and the movements of the globe prevent any notable interference in vision, even 

 with one eye. The sclerotic coat is for the protection of its contents and for the inser- 

 tion of muscles. The iris has an action similar to that of the diaphragm in optical instru- 

 ments. The suspensory ligament of the lens, the ciliary body, and the ciliary muscle, are 

 for the fixation of the lens and its accommodation to distinct vision at different distances. 

 The choroid is a dark membrane for the absorption of light, preventing confusion of vision 

 from reflection within the eye. 



Refraction by the cornea is effected simply by its external surface. The rays of light 

 from a distant point are deviated by its convexity so that, if they were not again refracted 

 by the crystalline lens, they would be brought to a focus at a point situated about T \ of 

 an inch behind the retina. Without the crystalline lens, therefore, distinct, unaided 

 vision is generally impossible, although the sensation of light is appreciated. In cases of 

 extraction of the lens for cataract, the crystalline is supplied by a convex lens placed 

 before the eye. 



The rays of light, refracted by the anterior surface of the cornea, are received upon 

 the anterior surface of the crystalline lens, by which they are still farther refracted. 

 Passing through the substance of the lens, they undergo certain modifications in refrac- 

 tion dependent upon the differences in the various strata of the lens. These modifica- 

 tions have not been accurately calculated ; but it is sufficient to state that they contribute 

 to the accuracy of the formation of the retinal image and to the production of an image 

 practically free from chromatic dispersion. As the rays pass out of the crystalline lens, 

 they are again refracted by its posterior curvature and are brought to a focus at the area 

 of distinct vision. 



The rays from all points of an object distinctly seen are brought to a focus, if the 

 accommodation of the lens be correct, upon a restricted surface in the macula lutea ; but 

 the rays from different points cross each other before they reach the retina, and the 

 image is consequently inverted. This is a fact capable of actual demonstration, as we 

 have shown in treating of the formation of images in the eye. 



Calculating the curvatures of the refracting surfaces in the eye and the indices of 

 refraction of its transparent media, it has been pretty clearly shown, by mathematical 

 formula, that the eye, viewed simply as an optical instrument, and not practically, as the 

 organ of vision, presents a certain degree of spherical and chromatic aberration; but with 

 these formulae we have little to do in our purely physiological consideration of vision. 



In most calculations of the size of images, the positions of conjugate foci, etc., in nor- 

 mal and abnormal eyes, a schematic eye reduced by Bonders, after the example of List- 

 ing, is regarded as sufficiently exact for all practical purposes. This simple scheme 

 represents the eye as reduced to a single refracting surface, the cornea, and a single liquid 

 assumed to have an index of refraction equal to that of pure water. The distance between 

 what are called the two nodal points and between the two principal points of the dioptric 

 system of the eye is so small, amounting to hardly T 7 of an inch, that it can be neglected. 

 In this simple eye, we assume a radius of curvature of the cornea of about of an inch, 

 and have a single optical centre situated ^ of an inch back of the cornea, the " principal 

 point " being in the cornea, at the axis of vision. The posterior focal distance, that is, 

 the focus, at the bottom of the eye, for rays that are parallel in the air, is about -f of an 

 inch. The anterior focal distance, that is, for rays parallel in the vitreous humor, is about 

 -f of an inch. The measurements in this simple schematic eye can be easily remembered 

 and used in calculations. 



Astigmatism. 



We have already alluded to an important peculiarity in the optical apparatus; which 

 is that the visual line does not coincide exactly with the axis of the eye. There is still 



