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 Cumming 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 ;f 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 Helmholtz's 

 instrument ; the opaque perforated reflector forming the basis of that of Coccius ; 

 the whole reflection is supposed to take place from the surface of the retina. 



* See Helmholtz's " Besclireibung ernes Augen-Spiegels," &c. ; and Dr Sanders' excellent ab- 

 stract of this Memoir, from which I have borrowed in the text ; also Ruete's Preliminary Chapters 

 in his Bildliche Darstellung. 



f On the Luminousness Observed in the Eyes of Human Beings. Edinburgh Phil. Journal, 

 1827, p. 164. 





