VISION. 



1441 



upon the cornea, are refracted by the trans- 

 parent media of the eye in proportion to the 

 difference between the density of these media, 

 and that of the air, and in proportion to the 

 curves presented by their several surfaces. It 

 is of course the central ray only, or that 

 which passes through the axis of the eye, 

 that is not refracted ; all the other rays un- 

 dergo refraction, and are approximated to the 

 central ray. The prime rays have been termed 

 rays of direction, because every prime or axial 

 ray determines the direction of the other rays. 

 As every object emits rays, from every point, 

 in all directions, which rays then proceed in 

 straight lines, it necessarily follows that, un- 

 dergoing these refractions, there must be some 

 point in the eye at which the axial rays of 

 the different pencils, proceeding from the ob- 

 ject, cross ; and this appears to be very near 

 the centre of the eye, somewhat behind the 

 crystalline. Sir D. Brewster places it in the 

 geometric centre of the e\e-ball, consequently 

 a little within the crystalline lens ; and Volk- 

 mann has described the point of intersection 

 as being 3'" 97 behind the cornea, 0'" 43 before 

 the posterior surface of the crystalline, and 

 6"' 23 in front of the retina. Other and very 

 careful observers, however, place it at a very 

 bhort distance behind the crystalline lens. It is 

 called the focal centre, and the angle formed 

 by the intersecting axial rays from two points 

 is the visual angle. This focal centre seems, 

 according to the researches of Ruete, to be 

 of importance in another way. In the steady 

 contemplation of objects, we have to bring 

 them into the focal centre of the produced 

 visual axis ; and in the motion by which this 

 is accomplished, it would appear that the eye 

 revolves accurately round a point, which point 

 of revolution is the focal centre of the eye. 

 In vision, the muscles of the two eyes act 

 under the influence of the will, with a remark- 

 able and admirable sympathy; and it is on 

 this harmonious consent, as it were, of op- 

 posing muscles, that vision in its most perfect 

 form depends. 



Vision under water is attended with some 

 curious consequences, the result of what is 

 termed " internal" reflection. An eye placed 

 under perfectly still water, as for instance, the 

 eye of a diver, will see external objects only 

 through a circular aperture (as it were) of 

 96 55' 20" in diameter overhead. But ail 

 objects down to the horizon will be visible in 

 this space; those near the horizon being much 

 distorted and contracted in dimensions, espe- 

 cially in height. Beyond the limits of this 

 circle will be seen the bottom of the water, 

 and all subaqueous objects reflected and as 

 vividly depicted, as by direct vision ; and in 

 addition, the circular space above mentioned 

 will appear surrounded with a rainbow of faint 

 but delicate colours. In the eyes of fishes, the 

 humours being nearly of the refractive density 

 of the medium in which they live, the action 

 of bringing the rays to a focus on the retina is 

 almost entirely performed by the crystalline 

 lens, which is nearly spherical and of small 

 radius in comparison with the whole diameter 



VOL. iv. 



of the eye ; there is also a very great in- 

 crease of density towards the centre, whereby 

 spherical aberration is obviated, the corneal 

 refraction having little influence. 



When speaking of light we have mentioned 

 spherical and chromatic aberrations; and it is 

 necessary that they should be again alluded to 

 in reference to the eye. Spherical aberration is 

 beautifully counteracted by the figure and vary- 

 ing density of the crystalline lens; which, in- 

 creasing in refractive power towards the centre, 

 refracts the central rays in each pencil, to the 

 same point as its external rays: but an important 

 agent in obviating this aberration is the iris, 

 which is, as it were, perpetually on the watch 

 to limit the rays entering the eye to those 

 which produce a perfect image, cutting off 

 others which, by their obliquity of incidence, 

 might occasion the imperfection in question. 

 Opticians endeavour to obtain the same effect 

 in their instruments by the employment of an 

 opaque screen or diaphragm ; but no device 

 of human art can equal the ever changing 

 pupil of the living eye. 



Sir David Brewster, and some other autho- 

 rities, deny that the eye is perfectly free from 

 chromatic aberration : and it is certain that 

 when the pupil is dilated by belladonna, and 

 the lateral rays freely admitted, coloured 

 fringes are perceptible, as we have ourselves 

 experienced ; on the other hand, the forms 

 and relative densities of the humours of the 

 eye closely imitate the achromatic combina- 

 tion of lenses, for the two menisci, formed by 

 the aqueous and vitreous humours, having 

 the double convex crystalline lens, of greater 

 density than either, placed between them, 

 fulfil these conditions very happily, and can 

 hardly fail to obviate, in a material degree, 

 chromatic aberration. The coloured fringes 

 we have spoken of, as being produced by the 

 dilatation of the pupil, must not be confounded 

 with the chromatic images, which depend on 

 certain conditions of the retina. The former 

 are always connected with refraction, and 

 they attach particularly to two conditions, 

 namely, the falling of a light close to a shadow, 

 and the projection of the limits of either on 

 the retina, in such a way that all sharpness of 

 outline is lost. The transition from light to 

 shade the blending of the light and darkness 

 thus produced gives rise to coloured fringes. 

 This may be shown by taking up a position, 

 at the end of a room, before a brightly illu- 

 minated window, and holding up any small 

 object, such as a pencil, before the eye, which 

 must be steadily fixed upon the window- 

 sash. Presently prismatic colours appear on 

 either side of the bars as well as of the 

 pencil. Goethe explained all chromatic phe- 

 nomena on the sole ground of modifications 

 in light and shade. In the light seen through 

 dull media, according to his theory, yellow 

 is first perceived, then in succession, red, 

 pale blue, blue, violet, black blue, and black ; 

 and he explains dioptrically-formed coloured 

 margins from the subjective side, by a mutual 

 encroachment of the light and dark, the shadow 

 before the light being perceived as blue and 



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