1440 



VISION. 



each from the lens. If the object lie twice 

 as far as the focal distance from the lens, the 

 image will be formed on the other side at an 

 equal distance, consequently the image and 

 the object are equal in size. But if the object 

 approach nearer to the glass, the image will 

 recede, becoming larger. We thus obtain in- 

 verted and enlarged images of objects stand- 

 ing further from the lens than the focal dis- 

 tance, yet not so far as twice that distance. 

 Thus the image A'B' is larger than the object 

 AB. If the object be further removed from 

 the glass than twice the local distance, the 

 image will be nearer ; we therefore obtain in- 

 verted diminished images of distant objects. 



Dioptric phenomena. Krause has described 

 the anterior surface of the cornea as being 

 spherical, the posterior parabolic. We re- 

 cently had an opportunity of examining a 

 human cornea in less than an hour aftertheeye 

 had been extirpated from the living subject, 

 and satisfied ourselves that, in that case at 

 least, the two surfaces were perfectly parallel. 

 According to Chossat the figure of the cornea 

 is an el'ipsoid of revolution about the major 

 axis, which axis determines the axis of the 

 eye. The ratio of the semi-axis of this ellipse 

 to the excentricity he determines at 1'3 ; and 

 this being nearly the same with the index of 

 refraction, parallel rays incident on the cornea 

 in the direction of its axis will be made to 

 converge, with great exactness, to a focus si- 

 tuated behind it, the aberration, which would 

 have existed had the external surface been 

 of a spherical figure, being almost completely 

 destroyed. The form of the crystalline lens 

 is that of a solid of revolution, having its 

 anterior surface much less curved than its 

 posterior: by some authorities both surfaces 

 are described as being ellipsoids of revolution 

 about their lesser axes ; but Krause found the 

 posterior surface to have a parabolic curve, 

 whose parameter was from 3| w to 5'". lie 

 found al.iO that the elliptic. il curve of the an- 

 terior surface of the lens varied as regards its 

 long axis, in different cases from 4"' to 5-^"' ; 

 its short axis also varying from If"' to V.\'". 

 Dr. Albert de Gra'efe informs us that he has 

 repeated these observations, and ascertained 

 them to be correct. Valee states that he lias 

 found by a comparison of Krauss's measure- 

 ments ot' the dimensions of the eye, that the 

 exterior convex surfaces have exactly that 

 geometrical form which produces foci free 

 from deviations. lie calls them optoidal sur- 

 faces, and also finds that the posterior convex 

 surfaces are at least so far optoidal, as the 

 pencils of light penetrating into the eye in- 

 fringe upon them. The axes of the two sur- 

 faces of the lens are not exactly coincident in 

 direction with each other, nor with that of the 

 cornea. 



The refractive index of the surface of the 

 lens is, according to Brewster, 1 '3767 ; of 

 the centre P3990, the mean being P3839 : 

 that of the vitreous humour is P3394 ; thus 

 the refractive density of the lens being greater 

 than that of either the aqueous or vitreous 

 humour, it exercises an important influence 



on the converging rays incident on it from 

 the cornea. The effect of its elliptic figure 

 is probably to correct the aberration of ob- 

 lique pencils, and general aberration is still 

 further obviated by the peculiar and varying 

 density of its substance. By Professor James 

 Forbes the variable density is supposed to 

 alter the figure of the lens under pressure, and 

 so to assist in focal adjustment. 



As the rays refracted by the aqueous hu- 

 mour pass into the crystalline, and those from 

 the crystalline into the vitreous humour, the 

 indices of refraction of the separating sur- 

 face of these humours will be, according to 

 Brewster : 

 From aqueous humour to outer layer 



of crystalline - - - - P04GG 

 From ditto to crystalline, using the 



mean index - - - P0353 



From vitreous to crystalline outer 



layer - P0445 



From ditto to ditto, using the mean 



index - P0332 



Suppose, then, the eye to be directed to- 

 wards an object, the rays of light proceeding 

 from that object are thus disposed of. Those 

 which strike the cornea very obliquely are 

 reflected, as are those which impinge upon 

 the sclerotic ; a large proportion of the rays, 

 however, pass through the cornea, being power- 

 fully refracted by it, and the aqueous mem- 

 brane, and less by the aqueous humour 

 behind : proceeding onwards, many of the 

 rays are arrested by the iris, some being ab- 

 sorbed, others reflected, conveying the colours 

 and brilliancy characteristic of that .membrane. 

 The more central rays pass through the pupil 

 and the crystalline lens. The layers of this 

 body, increasing in density from the circum- 

 ference to the centre, resemble in their effect 

 upon the rays the atmosphere of this earth, 

 which causes a gradual bending of the ra^s 

 flowing from the heavenly bodies ; so the crys- 

 talline lens by its form and structure gradually 

 but powerfully increases the convergence of the 

 rays which penetrate it, both on their entra*nce 

 and their exit. They then traverse the vitreous 

 humour, the chief use of which appears to 

 be to afford support to the expanded retina, 

 and are brought in a perfectly natural eye, 

 to foci upon the retina, forming there an 

 exact but inverted picture of the object. If 

 the eye of a white rabbit or any other albino 

 be carefully cleansed, the flame of a taper held 

 before the cornea may be seen inverted at the 

 back of the eye, increasing in size as the taper 

 is brought near, diminishing if it retires, and 

 ever moving in the direction opposite to that 

 given to the flame. 



Such, in general terms, are the phenomena 

 attending the formation of images in the eye; 

 and they are strictly in accordance with the 

 mode of action of convex lenses, of which 

 the rule is, that an image formed by a convex 

 lens is inverted, and its position relatively to 

 the position of the object and its magnitude, 

 are to that of the object as its distance from 

 the lens is to the distance of the object from 

 the lens. The rays composing a pencil falling 



