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 in old age as to diminish the transparency, when it appears opaque or milky, or resembling the semitransparent 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 he crushed between the finger and thumb, and in old age becomes tough and firm. Ilence 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, " 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 those from the crystalline into the vitreous humour, the indices of refraction of the sepa rating surface of these humours will be, from the aqueous humour to the outer coat of the crystalline 1.0466, from the aqueous humour to the crystalline, using the mean index, 1.0353, from the vitreous to the outer coat of the cry stalline 1.0445, from the vitreous to the crystal line, using the mean index, 1.0332." Dr. Young says, " On the whole it is probable that the refractive power of the centre of the human crystalline, in its living state, is to that of water nearly as 18 to 7; that the water im bibed after death reduces it to the ratio of 21 to 20; but that on account of the unequable den sity, its effect in the eye is equivalent to a refraction of 14 to 13 for its whole size." Respecting the chemical composition of the lens, Berzelius observes, that " the liquid in its cells is more concentrated than any other in the body. It is completely diaphanous and colourless, holding in solution a particular animal matter belonging evidently to the class of albuminous substances, but differing from fibrine in not coagulating spontaneously, and from albumen, inasmuch as the concentrated solution, instead of becoming a coherent mass on the application of heat, becomes granulated exactly as the colouring matter of the blood when coagulated, from which it only differs in the absence of colour. All those chemical properties are the same as those of the co louring matter of the blood. The following are the principles of which the lens is com posed : peculiar coagulable albuminous matter 35.9, alcoholic extract with salts 2.4, watery extract with traces of salts 1.3, membrane form ing the cells 2.4, water 58.0.