REVIEWS. 
243 
and the practice of surgery has, besides others, its stethoscope and its Ophthal- 
moscope. A few words of explanation as regards the last-named, (an 
instrument which derives its designation from two Greek words, signifying 
“to behold,” and “the eye,”) will probably be interesting to some of our 
readers. 
First, however, it will be necessary for us briefly to consider the struc- 
ture of Nature’s wondrous and perfect instrument, which is in this case the 
active as well as the passive agent — the observer and the object of in- 
vestigation, — we mean the human eye, — and then we shall be able to 
understand more clearly the mode in which the other instrument is 
applied. 
The eye has been aptly compared to the “ camera ” of a photographic 
artist, and we shall find that a survey of its parts fully bears out the 
analogy. 
The whole of the posterior portion (see 
woodcut, from a a) backwards constitutes 
the box or dark chamber, the walls of which 
are composed of an outer membrane of a 
milk-white colour and firm fibrous consist' 
ency (the sclerotcia, b, b), and of an inner 
one (the choroid, c, c ), in which the colouring 
matter (pigmentum nigrum) is deposited, 
that darkens the inner portion of the cham- 
ber. The last-named is however not empty 
and hollow as in the camera, but it is filled with a jelly-like transparent 
substance ( d ) (the vitreous humour), which allows a free passage to the rays 
of light. Anterior to (« a) the chamber, we have the analogue of the brass 
tube and lenses of the camera. There is the external lens (the cornea, e), 
between which and the “ crystalline ” lens (f) we find the curtain or dia- 
phragm ( g ff), possessing, in the eye, that remarkable power of contraction 
and dilatation whereby more or less light is admitted, as circumstances 
may necessitate, and which is called the “iris,” in consequence of the 
hues imparted to it in different individuals by its contained colouring 
matter. Between the outer lens (cornea) and the inner one (crystalline 
lens), is the “ aqueous humour,” a limpid fluid which fills the anterior 
chamber ; and, lastly, as the reader is well aware, there is the aperture in 
the iris or diaphragm — the window of the eye we might term it, if it were 
not better known as the “pupil being so called on account of the little 
image which it reflects of him who examines it. 
We have thus all the essential parts of the camera ; or, to speak more 
correctly, the camera has many of the essential parts of the eye (for wa 
have only named a few of the most prominent) ; and now we require in the 
latter, the “sensitive plate,” which receives impressions of external objects. 
This is found in the retina (/<), a delicate membrane, lining the whole 
of the concave surface of the “ choroid,” and over which are distributed 
innumerable ramifications of the optic nerve ( i ), which pierces the back of 
the eyeball a little below the posterior extremity of its axis. The retina 
it is that receives the external image after it has passed through the 
several media already named, — the cornea, pupil, lens, and vitreous 
