REFRACTION OF LIGHT IN THE EYE. 437 



tion ; certain other fluid and solid refractive media, viz., the aqueous 

 humor, a, crystalline lens, Z, and vitreous humor, v, are superadded ; 

 of these, the crystalline lens is the most important, and represents the 

 internal lens of the water camera obscura ; the perforated diaphragm 

 is represented by the iris, i, and pupil ; lastly, the retina, r, occupies 

 the position of the recipient surface or screen. To complete the com- 

 parison, when an object is placed in front of the eyeball, at a suitable 

 distance, an inverted image of it is projected on to the retina (see the 

 arrow and its image). This image cannot be seen in the living eye ; 

 but it may be demonstrated in the human eye, arid in the eyes of the 

 larger quadrupeds, taken out after death, on removal of a part of the 



Fig. 83. 



Fig. 83. Diagrammatic section of the eyeball, showing the position of its parts, and the mode of forma- 

 tion of the inverted image of an object on the retina, at the back part of the eyeball, c, the cornea, s, 

 sclerotic, e. the choroid, 6, the ciliary processes, r, the retina, a, the aqueous humor. /, the crystalline 

 lens, r, the vitreous humor, t, the iris. The position of the ciliary ligament, from which the ciliary 

 muscle takes its origin, is at the junction of the cornea, sclerotic, and iris, h the optic nerve. The arrow, 

 with the lines representing pencils of light, and the inverted arrow on the inside of the back of the eyeball, 

 may be compared with the same parts in Diagram I, p. 434. 



sclerotic and choroid coats from the back of the eyeball, and even 

 without such dissection, in white rabbits, and other small albino quad- 

 rupeds, in which the coats of the eyeball are transparent. 



But the eyeball differs, as we shall see, from an artificial camera 

 obscura, in many ways. Its form is globular, not cubical, so that its 

 screen presents a concave surface ; its chief refracting medium, the 

 crystalline lens, is capable of special adjustment for objects at differ- 

 ent distances ; the eye is also corrected for the aberrations of ordinary 

 lenses ; its diaphragm has a self-regulated aperture of variable size ; 

 and lastly, the recipient screen is a sensitive surface, which becomes 

 excited by the image thrown upon it, in such a definite manner, that 

 distinct and corresponding visual impressions are thereby produced in 

 the sensorium, conveying to the mind, impressions of light and shade, 

 form and color. 



The eye acts upon light like a compound lens ; it consists, indeed, 

 of a compound system of refracting media. Thus the cornea forms 

 a meniscus , the aqueous humor, a convexo-concave, the crystalline lens, 

 a double convex, and the vitreous body, a concavo-convex lens. The 

 refractive power, or index, of air being taken as 1.003, and that of 

 water, as 1.33, the refractive index of the cornea is 1.33, of the aque- 

 ous humor, 1.34, of the capsule and outer layers of the lens, 1.35, of 

 the succeeding layers of the lens, 1.38, of the nucleus or centre of 

 the lens, 1.41, and of the vitreous humor, 1.35. The mean refractive 



