322 PHYSIOLOGY CHAP. 



the eye. This was in fact the theory put forward by Boerhaave 

 and supported by Haller. But it is based upon an erroneous inter- 

 pretation of the facts. The fundus of the albino eye looks red, 

 not owing to absence of pigment, but because the light penetrates 

 not only through the pupil, but also through the sclerotic and 

 uvea, which are semi-transparent. Bonders found, on placing an 

 opaque diaphragm with an aperture of the same diameter as the 

 pupil in its centre in front of an albino eye, that the pupil 

 appeared black, as in normal pigmented eyes. The fundus of the 

 pigmented eye accordingly looks black because it is not sufficiently 

 illuminated, as light can only enter by the pupil, and the light 

 that enters and is not completely absorbed by the pigment leaves 

 in the same direction as that by which it entered, and may 

 consequently be intercepted by the head of the observer. 



Another argument against Boerhaave's theory is that the eyes 

 of many carnivora and nocturnal animals glitter in the dark. To 

 explain this it was assumed that the fundus of the eye in these 

 animals was phosphorescent like the photogenic organs of certain 

 insects, and served as a lantern to detect their prey. It was also 

 stated that this photogenic activity of the retina increased if the 

 animals were irritated. 



Prevost of Geneva (1810) first discovered the true ex- 

 planation of this phenomenon. He proved that the scintillation 

 in the eye of the dog, cat, etc., is due to the light reflected from 

 the tapetum cellulosum with which the fundus of the eye in these 

 animals is provided, and which acts as a mirror. The shimmer is 

 never seen in total darkness, but only in twilight, and it cannot be 

 provoked by the will nor by external stimuli. 



Gruithuisen confirmed the observations of Prevost, and added 

 the fact that the shining of the pupil may persist in dead animals. 

 The retina, therefore, has no photogenic power, and the phenomenon 

 depends on the light from without, which penetrates the eye and 

 is reflected from the fundus. 



As early as 1704, Mery observed that it was possible under 

 certain special conditions, as on examining an eye after placing 

 the head under water, to distinguish the vessels of the cat's retina 

 in full light through the pupil. In 1709 the celebrated geometer 

 and astronomer de la Hire, commenting on Mery's observation, 

 demonstrated that immersion in water abolishes corneal refraction 

 and apparently brings the fundus of the cat's eye nearer the 

 eye of the observer. The view of the retinal vessels is facili- 

 tated, as the pupils dilate in consequence of immersion and 

 the fundus is better illuminated. These important observations 

 of Mery and de la Hire long remained barren, and were not 

 utilised as the starting-point of the practical problem of ophthal- 

 moscopy, that is how, under ordinary conditions, to obtain a 

 distinct view of the fundus of the eye, both in the lower animals 



