TAPETA LUCIDA AND EYESHINE 229 



The light rays which are focused by the dioptric apparatus and pass 

 through the retina are never completely absorbed by the chorioidal pig- 

 ment. If they were, the ophthalmoscope would never have been possible. 

 With this instrument the observer looks along a beam of light which 

 is directed through the pupil of the eye of the subject. Enough of the 

 light is reflected from the subject's eyeground, directly back into the eye 

 of the observer, to enable the latter to see something of the retina and 

 the inner surface of the chorioid of the subject, magnified by the subject's 

 own cornea and lens. 



So bright a light as that of the ophthalmoscope does not often enter 

 the eye directly, and the fraction of more ordinary illumination which 

 reflects from the chorioid is too weak to blur the principal image and 

 detract from visual acuity. The photographer has to rely on essentially 

 the same phenomenon. He has a right to expect that the dead-black 

 lining of his camera will reflect practically no light through or upon the 

 film. When such reflection does affect the film due to some defect in the 

 camera, the picture is blurry with the unwanted light and the photo- 

 grapher calls the result 'halation'. 



Value and Basis of Eyeshine — There is one circumstance in which 

 one might conceivably strive to produce a very maximum of halation: 

 when the light-intensity is extremely low and a correspondingly length- 

 ened exposure is for some reason impossible. Cameras have occasionally 

 been built, in which the emulsion of the plate is on the back surface and 

 is in contact with a layer of bright mercury. This layer forms a mirror, 

 reflecting the light back through the emulsion and thus increasing its 

 effectiveness. 



When a biologist is asked to account for the phenomenon of 'eyeshine' 

 in animals he may give the flip explanation : "they do it with mirrors" — 

 and have every assurance that he is actually being perfectly matter-of-fact 

 and scientifically accurate. When we consider how brightly the eyes of 

 many animals reflect the light of our headlights as we drive past them at 

 night, it is apparent that these species must be reflecting light back 

 through their retinae instead of absorbing it in a typically pigmented 

 chorioid. Ophthabnoscopic and histological investigation bears out this 

 suspicion, and usually discloses a special mirroring device located some- 

 where behind the rod-and-cone layer. Though it is very differently con- 

 stituted in different cases, this mirror is generically called the tape turn 

 lucidum. This apt term means, literally, 'bright carpet'. The tapetum is 



