330 DR GEORGE WILSON ON THE 
depends upon the fact, that when rays of light enter the eye, and fall upon its 
back wall. as many of them as are reflected from the retina, or from the choroid 
behind it, will exactly retrace their course, and pass out through the pupil to the 
luminous body or illuminated object from which they came. Thus the diverging 
rays of a gas-flame are converged by the refracting media of the eye, to a focus 
upon the retina, where they unite to produce a picture, and thereafter in great 
part traverse that membrane and fall upon the choroid. If from either of these 
membranes rays are reflected (and for the sake of simplicity, we may, for the 
present, limit ourslvees to the retina, which is the more powerful reflector of the 
two), they will follow in a reversed direction, the very course which they took in 
reaching that membrane, and return to the gas-flame, producing there an image of 
the picture on the retina, so that the reflected image of the flame is placed upon, 
and coincides in size and position with the actual flame.* To see, therefore, into 
the deeper chambers of a living eye, we must arrange matters so that we can look 
along the straight line of the reflected rays, without intercepting the light from 
which they originally came. The earlier observations of Cummine were made 
without any special arrangement to prevent such interception of light ; they were 
rendered possible by the circumstance, that certain of the rays returning from 
the bottom of the eye undergo irregular reflection, and diverge from the direct 
line which theoretically all should follow, so that if the observer keeps a very 
little to the side of this line, standing almost between the light and the observed 
eye on which it is falling, he catches a sufficient number of the irregular rays to 
see into the interior of the eye from which they come. 
It was doubtless the accidental realization of this condition of matters, that 
led to the occasional observation, from very early times, of luminous emissions 
from human eyes; but even when the necessary conditions are fully realized, 
the illumination is so imperfect that the results are unsatisfactory. By the em- 
ployment, however, of a plane transparent reflector, such as a plate of polished 
glass, or of a plane or concave mirror, perforated or rendered transparent at the 
centre, the source of the light may be placed at an angle, both to the observing and 
the observed eye, so as to enable the former to receive directly much of the light 
reflected from the latter. The following diagram will show how this occurs in 
the case of the transparent reflector, which is the essential part of HrtMHOLT2’s 
instrument; the opaque perforated reflector forming the basis of that of Coccrus; 
the whole reflection is supposed to take place from the surface of the retina. 
* See Hermuonrz’s “* Beschreibung eines Augen-Spiegels,” &c.; and Dr Sanpers’ excellent ab- 
stract of this Memoir, from which I have borrowed in the text; also Ruxrrez’s Preliminary Chapters 
in his Bildliche Darstellung. 
: ! On the Luminousness Observed in the Eyes of Human Beings. Edinburgh Phil. Journal, 
827, p. 164. 
