OPTICS. 



41 



Fig. 42 



a vertical section in (fig. 42.), is nearly 

 of a globular form, with a slight elonga- 

 tion or projection in front. It consists 

 of four coats or membranes, viz. the 

 Sclerotic, the Cornea, the Choroid, and 

 the Retina ; of two fluids or humours, 

 the Aqueous and the Vitreous ; and of 

 one lens, called the Crystalline. The 

 Sclerotic coat, a a a, (fig. 42.), is the 

 outer and strongest coat, to which the 

 muscles for giving it motion are attached. 

 It constitutes the white of the eye, a a, 

 (fig. 41.) It is joined to the Cornea, b b, 

 or the clear and transparent circular 

 membrane through which we see. The 

 Cornea, which is equally thick through- 

 out, is very tough, and consists of seve- 

 ral layers or folds to give it strength, so 

 as to defend the delicate parts within 

 from external injury. On the inner sur- 

 face of the sclerotic coat is a delicate 

 membrane, called the Choroid coat, 

 which is covered with a black pigment. 

 On the inner side of this lies the 

 Retina, r r r r, which is the innermost 

 coat, and is a tender reticular mem- 

 brane, formed from the expansion of 

 the optic nerve, which enters the eye at 

 O, a little more than one-tenth of an inch 

 from the axis on the side towards the 

 nose. At the end of the axis of the 

 eye, and in the very centre of the 

 retina, there is a small hole, with a 

 yellow margin. It is called the foramen 

 centrale, or central hole, though it is not 

 a hole, but merely a transparent spot, 

 free of the soft pulpy matter of which 

 the retina consists. 



A flat membrane of a circular form, 

 ef, called the iris, (cc, fig. 41.), and seen 

 through the cornea b b, divides the in- 

 terior globe of the eye into two very un- 

 equal parts. It has a circular opening, 

 d,Jig. 41, in its centre, called the pupil, 

 which expands when the light which 

 enters the eye is diminished, and con- 

 tracts when the light is increased. The 

 space before the iris t called the anterior 



chamber of the eye, contains the aque- 

 ous humour, from its resemblance to 

 pure water ; and the space behind the 

 iris is called the posterior chamber, and 

 contains the crystalline lens, cc, and 

 the vitreous humour, which fills all the 

 rest of the eye. The crystalline lens is 

 suspended in a transparent capsule, or 

 bag, by what are called the ciliary pro- 

 cesses, gg. This lens is more convex 

 behind than in front, as the figure shows; 

 and it consists of concentric coats com- 

 posed of fibres. It increases in density 

 from its circumference to its centre, for 

 the purpose of correcting its spherical 

 aberration. The vitreous humour, V V, 

 occupying the largest portion of the 

 eye, lies immediately behind the crystal- 

 line lens, and fills the whole space be- 

 tween it and the retina, rrrr. 



The following are the dimensions of 

 the eye, as given by Dr. Young and 

 M. Petit: 



English Inches 



Length of the optical axes 0.91 



Vertical chord of the cornea 0.45 



Versed sine of ditto 0.11 



Horizontal chord of the cornea 0.47 



Opening of pupil seen through the cornea 0.27 to 0.13 

 Diminished by magnifying power of cornea to 



0-25 to 0.12 

 Radius of the anterior surface of the crystalline 



lens 030 



Radius of the posterior surface 0.22 



Principal focal distance of the lens 1.73 



Distance of the centre of the optic nerve from 



the central hole at the end of the axis 0.1L 



Distance of the iris from the cornea O.io 



Distance of the iris from the anterior surface of 



the crystalline 0.02 



Range of the eye, or diameter of field of vision 110 



Dr. Brewster and Dr. Gordon took the 

 following measures of the crystalline 

 and cornea from the eye of a woman 

 above 50 years of age, a few hours after 

 death. 



Diameter of the crystalline 0.378 



Diameter of the cornea 0.400 



Thickness ol the crystalline 0.172 



Thickness of the cornea 0.042 



The following are the refractive pow- 

 ers of the humours of the eye, according 

 to different observers : 



Aqueous 

 Humour. 



Hauksbee 1.335S*5 

 Jurin.... 1.3333 

 Kochon.. 1.329 

 Young .. 1.3333 

 Brewster 1.336t5 



Crystalline Lens. 

 Outer Coat Centre. Mean. 



1.3767 1.3990 1.3S39 



Vitreous 

 Humour. 

 1.33595 



1.332 

 1.3394 



From the last of these measures we 

 may deduce the following indices of re- 

 fraction : 



Index of Refraction. 

 For rays parsing from the aqueous humour 



into the outer coat of the crystalline lens 1.0466 

 For rays passing from the aqueous humour 



into the crystalline, taking its mean index 



of refraction 1.0353 



For rays passing from the outer coat of the 



crystalline into the vitreous humour .... 0.93 



