The problem to be solved consists, then, in the simple illumination of the eye to be observed by a source of light so arranged that the observer can be placed in the axis of the rays entering and emerging without intercepting those rays. This may be most conveniently effected by placing the source of light aside of the eye to be observed, and observing through a pierced concave mirror, which reflects that light into the eye. We can then, by looking through the central aperture of this mirror, place ourself in the path of the entering and emerging rays. The mirror becomes the source of light to the observed eye; the rays which it flashed into the eye emerge {122} in part, and return along the same path, forming the aerial image at a distance and under circumstances regulated by the optical conditions of the eye observed, and within view of the observer who is looking through the mirror. A very simple diagram will suffice to explain this: _r a_ is the circle of diffusion of the retina, and the lines indicate how the reflected rays will pass through the media of the eye, and form at _r' a'_ real enlarged but inverted image of the fundus of the eye. This will be placed at the distance of distinct vision of the subject, and has relation to the accommodation of the eye. [Illustration: Diagram of preceding discussion.] As these are variable quantities, the practice of ophthalmoscopy demands a little address, which habit quickly gives. It is for want of understanding this, and from impatience of these preliminary difficulties, that many have been discouraged at the outset, and have abandoned unwisely the attempt to learn the use of the ophthalmoscope. The image obtained in the way mentioned is not so distinct as to give that full perception of details which is necessary for scientific and medical purposes. A more defined image is obtained by interposing, for example, a bi-convex lens on the path of the luminous rays emerging from the eye observed. The effect of holding such a lens of short focus before the observed eye whilst examining it with a concave ophthalmoscopic mirror is to cause the rays emerging from the eye to undergo a further refraction, and to modify the actual image which they form, producing one which is smaller, more defined, but still inverted. This is the most simple and one of the most satisfactory methods of exploring the eye with the ophthalmoscope. It is that of the most general and easy application, and I will, therefore, add a few words to explain how it may most conveniently be practised.