338 
OBSERVATIONS ON SOUNDNESS. 
of light being incident upon them from air:—Aqueous 
humour, 1*336(3. Crystalline lens at 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 crys¬ 
talline and those from the crystalline into the vitreous 
humour, the indices of refraction of the separating surface of 
these humours will be—From aqueous humour to outer 
coat of the crystalline, 1*0466. From same to crystalline, 
using the mean index, 1*0333. From vitreous to crystalline 
outer coat, 1*0443. From same to same, using the mean 
index, 1*0332. 
We are further told by Dr. Golding Bird :—The eye in all 
warm-blooded animals is formed upon the type of man, with 
the occasional addition of supplementary structures, better 
fitting the organ for the performance of vision in the par¬ 
ticular animal. In fishes, residing in a medium of nearly 
the same refractive index as the aqueous humour, the latter 
fluid becomes useless, and is replaced by a viscid secretion of 
greater refractive power. The crystalline lens is in these 
animals nearly spherical, and placed behind the cornea, and 
the iris, which is close to the latter, is undilatable. In 
insects the eye is simple, consisting of a lenticular cornea, 
placed in front of a nervous expansion. When an object is 
viewed with both eyes in a healthy condition, it appears 
single, whilst it is obvious that a distinct image is painted 
upon each retina. This is readily explained by the fact, that 
the two images lying exactly in the direction of the optic 
axis overlap each other, and virtually produce but one 
impression. If the optic axis be not brought to coincide at 
the place of the object, the two images are separated, and 
then, as in the case of squinting persons, the object appears 
double, or confused.. 
‘‘The variation of focal length of the eye has been attributed 
by some physiologists to an alteration in the form of the 
crystalline lens by the contraction of its own fibres; but the 
structure of muscular fibre is so completely identical, from 
whatever part of the animal kingdom it may have been 
obtained, and so essentially different from the fibrous struc¬ 
ture of the lens, that it appears difficult to conceive the 
existence of muscularity in that organ; moreover, in all the 
higher orders of animals, the muscular structures are co¬ 
piously supplied with blood-vessels and nerves, neither of 
which have been detected in the crystalline lens. In order 
that vision may be distinct, it is necessary that corresponding 
points of the object and of the retina should be conjugate 
foci of the eye; or, in other words, that the pencils of rays 
