INSIDE THE EYE. 459 
is^ and wliat are some of tlie most useful results whicli have 
been obtained by its use. 
Let me first remind the general reader that in the human 
eje, behind the pupillary aperture of the coloured iris, which 
presents to the unaided eye of the observer the mere aspect of 
black darkness, lies first a clear bi-convex lens ; and behind 
this, filling the eye, and giving to it the character of a solid 
ball, a transparent globular mass, known as the vitreous body 
or humour. It is into a depression in the front of this that 
the aforesaid lens is fitted, so that the whole space of the eye 
behind the iris is filled by the lens and vitreous body. The 
optic nerve, or nerve of sight, which pierces the tunics of the 
eye ^t the back and near the centre, spreads out and forms an 
expanded tunic of nerve- structure which enwraps the vitreous 
body as far as its most forward edge, where the coloured iris 
descends in front of it. Enwrapping again this nerve-tunic 
or retina, is a vestment chiefly made of blood-vessels, con- 
nected by fine tissue and thickly coated with black pigment, 
having its own optical uses. This second outer pigmented 
vascular tunic is the choroid. This again is enclosed within the 
external strong fibrous membrane, which includes and protects 
all the sclerotic membrane {aKXrjpog, hard). These are the 
two humours and three tunics of the eye which can to a 
greater or less extent be examined during life by the aid of 
the ophthalmoscope. 
They can all be more or less investigated in the living eye 
by the aid of the ophthalmoscope, because by the aid of this 
instrument we are able to see through the pupillary space. 
If one considers what is the reason of the apparent darkness 
of the pupillary aperture and the chambers of the eye behind 
it, it is not difficult to gain an idea of the means by which this 
optical condition may be altered so as to enable us to see where 
all seems to the unaided vision obscure. 
This darkness of the pupillary aperture is attributable partly 
to obvious causes, such as the natural contraction of the pupil 
or iris, which occurs under light — this contraction limiting the 
number of rays which can enter the eye. Then that black 
pigment which lines the iris absorbs a great deal of light ; and 
thus, as in the case of albinos, whose eyes are deficient in 
pigment, or where the pupil is dilated, either through disease 
or by artificial agents, these obstacles for seeing into the living- 
eye are removed. But still the main difficulties are not cleared 
away ; and if you take for example an albino animal, such as 
one of those beautiful little white-furred rabbits, whose rosy 
eyes look like fiery opals edged with swanks down, and dilate 
the pupils with atropine, it is still not possible to see clearly the 
details of the structure within and at the back of the eye. 
