INSIDE THE EYE. 
461 
source of light to the observed eye ; the rays which it flashes 
into the eye emerge 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 u 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' o! 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. 
As these are variable quantities^ the practice of ophthalmo- 
scopy demands a little address, which habit quickly gives. It 
is for want of nnderstanding 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 deflned image is ob- 
tained 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 in- 
verted. This is the most simple and one of the most satis- 
factory 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 con- 
veniently be practised. 
We will suppose that it is the human eye which is to be 
examined. The room is to be made dark ; the person to be 
seated ; a light — the white flame of an oil-lamp or an Argand 
gas-burner — to be placed near his head, on the side, and 
at the level of the eye to be observed. The observer takes 
then the concave mirror in the hand, of the side towards the 
lamp, and placing it against the front of his eye, so that the upper 
edge rests against his eyebrow, brings his head to the level of 
that of the person seated, looks through the central perfora- 
tion at the eye to be observed, and by a little careful change 
in the direction of the mirror casts by its aid upon the eye 
examined the light of the lamp. 
He will now perceive that the pupillary aperture is illumi- 
nated, and, no longer black, shines with a silvery or reddened 
light. He takes now the bi-convex lens of short focus, in the 
