196 



EYE. 



presents greater curvatures does not require 

 concave glasses, as the old person requires con- 

 vex ones. The state of the eye, after the re- 

 moval of the lens by operation for cataract, 

 proves that it is a part of the organ essentially 

 necessary for correct vision. When the eye is 

 in other respects perfect, without any shred of 

 opaque capsule, any irregularity or adhesion of 

 the pupil, or any alteration in the curvature of 

 the cornea, as in young persons who have had 

 the lens properly broken up with a fine needle 

 through the cornea, vision is so good for distant 

 objects, that such persons are able to pursue 

 their common occupations, and walk with safety 

 through crowded streets, but they require the 

 use of a convex lens, of from three and a half to 

 five inches focus, for reading or vision of near; 

 old persons, however, generally require convex 

 glasses on all occasions after the removal of the 

 lens. That the curvatures of the lens are fre- 

 quently different in different individuals may 

 be inferred from the frequency of short sight, 

 or defective power of adaptation, not attributa- 

 ble to any peculiarity of the cornea. Petit 

 states that he found lenses of which the two 

 convexities were equal, and others of which the 

 anterior was greater than the posterior, and 

 more than once, one more convex on its ante- 

 rior surface in one eye, while that in the other 

 eye was in a natural state. He also occasion- 

 ally found the lens as convex in the advanced 

 period of life as in youth. I have repeatedly 

 observed the perfection of vision and power of 

 adaptation much greater in one eye than the 

 other in the same individual, without any defect 

 of the cornea, pupil, or retina ; and occasionally 

 have found young persons requiring the com- 

 mon convex glasses used by persons advanced 

 in life, and old persons becoming near-sighted, 

 and requiring concaves. The annexed letters 

 shew the difference of curvature at the different 

 periods of life, as represented by Sbmmerring. 

 A is the lens of the foetus; B, that of a child of 

 six years of age ; and C, that of an adult. 



The colour of the lens is also different at 

 different periods of life. In the foetus it is 

 often of a reddish colour; at birth and in in- 

 fancy it appears slightly opaque or opaline ; in 

 youth it is perfectly transparent; and in the 

 more advanced periods of life acquires a yel- 

 lowish or amber tint. These varieties in colour 

 are not visible, unless the lens be removed 

 from the eye, until the colour becomes so deep 

 an old age as to diminish the transparency, 

 when it appears opaque or milky, or resembling 

 the semi transparent horn used for lanterns. The 

 hard lenticular cataract of advanced life appears 

 to be nothing more than the extreme of this 

 change of colour, at least when extracted and 

 placed on white paper it presents no other 

 disorganization ; but the lens of old persons, 

 when seen in a good light and with a dilated 

 pupil, always appears more or less opaque, al- 



though vision remains perfect. The depth of 

 colour is sometimes so great, without any 

 milkiness or opacity, that the pupil appears 

 quite transparent although vision is lost. This 

 is perhaps the state of lens vaguely alluded to 

 by authors under the name of black cataract. 



The consistence of the lens varies as much 

 as its colour. In infancy it is soft and pulpy, 

 in youth firmer, but still so soft that it may be 

 crushed between the finger and thumb, and in 

 old age becomes tough and firm. Hence it is 

 that in the earlier periods of life cataracts may 

 be broken up completely into a pulp, and 

 absorbed with certainty, while in old persons 

 they adhere to the needle, unless very deli- 

 cately touched, and are very liable to be de- 

 tached from the capsule and thrown upon the 

 iris, causing the destruction of the organ. On 

 this account, therefore, the operation of extrac- 

 tion must generally be resorted to in old per- 

 sons labouring under this form of cataract, 

 while the complete division of it with the 

 needle and exposure of the fragments to the 

 contact of the aqueous humour secures its 

 removal by absorption in young persons. It 

 must not, however, be forgotten that the softer 

 lenticular cataract occasionally occurs in ad* 

 vanced life. 



The crystalline lens is a little heavier than 

 water. Porterfield, from the experiments of 

 Bryan Robinson, infers that the specific gra- 

 vity of the human lens is to that of the other 

 humours as eleven to ten, the latter being 

 nearly the same as water; and Wintringham, 

 from his experiments, concludes that the den- 

 sity of the crystalline is to that of the vitreous 

 humour in the ratio of nine to ten; the spe- 

 cific gravity of the latter being to water as 

 10024 to 10000. The density of the lens is 

 not the same throughout, the surface being 

 nearly fluid, while the centre scarcely yields to 

 the pressure of the finger and thumb, especially 

 in advanced life. Wintringham found the spe- 

 cific gravity of the centre of the lens of the ox 

 to exceed that of the entire lens in the propor- 

 tion of twenty-seven to twenty-six. The re- 

 fractive power is consequently greater than that 

 of the other humours. On this head Mr. 

 Lloyd, in his Optics, says, " In their refrac- 

 tive power, the aqueous and vitreous humours 

 differ very little from that of water. The re- 

 fractive index of the aqueous humour is 1.337, 

 and that of the vitreous humour 1.339; that of 

 water being 1.336. The refractive power of 

 the crystalline is greater, its mean refracting 

 index being 1.384. The density of the crystal- 

 line, however, is not uniform, but increases 

 gradually from the outside to the centre. This 

 increase of density serves to correct the aber- 

 ration by increasing the convergence of the 

 central rays more than that of the extreme parts 

 of the pencil." Dr. Brewster, in his Treatise 

 on Optics, says, a I have found the following 

 to be the refractive powers of the different 

 humours of the eye, the ray of light being 

 incident upon them from the eye : aqueous 

 humour 1.336; crystalline, surface 1.3767, 

 centre 1.3990, mean 1.3839; vitreous humour 

 1.3394. But as the rays refracted by the 

 aqueous humour pass into the crystalline, and 



