580 



ON THE MECHANISM OF THE EYE. 



water; and, that the thickness of the cornea 

 is too equable to produce any, effect on the 

 focal distance. 



For determining the refractive power of 

 the crystalline lens by a direct experiment, I 

 made use of a method suggested to me by 

 Dr. Wollaston. I found the refractive power 

 of the centre of the recent human crystal- 

 line to that of water, as 21 to 20. The dif- 

 ference of this ratio from the ratio of 14 to 13, 

 ascertained from calculation, is i)robably 

 owing to two circumstances. The first is, 

 that, the substance of the lens being in some 

 degree soluble in water, a portion of the 

 aqueous fluid within its capsule penetrates 

 after death, so as sdmewhat to lessen the 

 density. When dry, the refractive power is 

 little inferior to that of crown glass. The 

 second circumjtance is the unequal density 

 of the lens. The ratio of 14 to 13 is founded on 

 the supposition of an equable density: but, the 

 central part being the most dense, tlie whole 

 acts as a lens of sm.iller dimensions: and it 

 may be found by calculation (M. E. 465.) 

 that if the central portion of a sphere be sup- 

 posed of uniform density, refracting as 21 to 

 20, to the distance of one half of the radius, 

 and the density of the external parts to de- 

 crease gradually, and at the surface to be- 

 come equal to that of the surrounding me- 

 dium, the sphere, thus constituted, will be 

 equal in focal length to a uniform sphere of 

 the same size, with a refraction of 16 to 15 

 nearly. And the effect will be nearly the 

 same, if the central portion be supposed to 

 be smaller than this, but the density to be 

 somewhat greater at the surface than that of 

 the surrounding medium, or to vary more ra- 

 pidly externally than internally. Or, if a 

 lens of equal mean dimensions, and equal fo- 

 cal length, with the crystalline, be supposed 



to consist of two segments of the external por- 

 tions of such a sphere, the refractive density 

 at the 'centre of this lens must be as 18 to 17. 

 On the whole, it is probable that the refrac- 

 tive power of the centre of the human crys- 

 talline, in its living state, is to that of water 

 nearly as 18 to 17; that the water, imbibed 

 after death, reduces it to the ratio of 21 to 

 20 ; but that, on account of the unequable 

 density of the lens, its effect in the eye is 

 equivalent to a refraction of 14 to 13 for its 

 whole size. Dr. Wollaston has ascertained, 

 the refraction out of air, into the centre 

 of the recent crystalline of oxen and sheep, 

 to be nearly as 143 to 100; into the centre of 

 the cr^'stalline offish, and into the dried crys- 

 talline of sheep, as 152 to 100. Hence, the 

 refraction of the crystalline of oxen, in water, 

 should be as 15 to 14: but the human cry- 

 stalline, when recent, is decidedly, less re- 

 fractive. 



These considerations will explain the in- 

 consistency of different observations on the 

 refractive power of the crystalline ; and, in 

 particular, how the refraction which I for- 

 me^Jy calculated, from measuring the focal 

 length of the lens*, is so much greater than 

 that which is determined by other means. 

 But, for direct experiments. Dr. WoUaston's 

 method is exceedingly accurate. 



When I look at a minute lucid point, such 

 as the image of a candle in a small concave 

 speculum, it appears as a radiated star, as a 

 cross, or as an unequal line, and never as a 

 perfect point, unless I apply a concave lens, 

 inclined at a proper angle, to correct the 

 unequal refraction of my eye. If I bring 

 the point very near, it spreads into a surface 

 nearly circular, and almost equably illumi- 

 nated, except some faint lines, nearly la a 

 • Phil. Trans. 1793. 174. 



