The optical instrument by which we are able to effect this inspection is known by the fitting title of the _Ophthalmoscope_ ([Greek text] the eye; [Greek text], I survey). With this instrument, the manner of using it, and its valuable applications, I am necessarily professionally much occupied in daily work; and as the editor of the "Popular Science Review" has requested me to give some plain account of the matter, I will endeavor to afford an untechnical statement of what the ophthalmoscope is, and what are some of the most useful results which have been obtained by its use. Let me first remind the general reader that in the human eye, behind the pupillary aperture of the colored 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 _humor_. 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 at 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 colored iris descends in front of it. Enwrapping again this nerve-tunic or _retina_ is a vestment, chiefly made of blood-vessels, connected 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 {120} ([Greek text], hard). These are the two humors 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 seem to the unaided vision obscure. [Illustration: Doctor looking through ophthalmoscope.] {121}