VOL. LXXXIX.] PHILOSOPHICAL TRANSACTIONS. 431 



observer. Philosophers also have paid particular attention to it, and have endeavoured 

 to investigate the cause. On this subject there have been 2 opinions: one, that 

 the illumination arises from the external light collected in the eye, and reflected; 

 the other, that there is a quantity of light generated in the organ itself. Professor 

 Bohn, at Leipsick, made experiments which proved, that when the external light 

 is wholly excluded, none can be seen in the cat's eye. These experiments were 

 favourable to the first opinion ; but the brightness of the illumination is so great, 

 that it appeared to exceed any effect which could be produced through the medium 

 of the retina; so that some other source of light was thought necessary to 

 account for the phenomenon: this circumstance gave support to the 2d opinion. 



To determine which of the 2 opinions was just, several experiments were insti- 

 tuted, under the direction of Mr. Ramsden, who also assisted in making them. 

 The truth of Professor Bonn's experiments was readily ascertained; it therefore 

 only became necessary to inquire, whether the external light was of itself capable 

 of producing so great a degree of illumination as that seen in the cat's eye. This 

 was attended with difficulty; for, when the apartment, in which the experiments 

 were made, was so much darkened that nothing but the illumination from the eye 

 was visible, the animal, by change of posture, or some other means, almost im- 

 mediately deprived the observers of all light from that source. This was found to 

 be the case, whether the cat, the tiger, or the hyena, was the subject of the 

 experiment. On the other hand, when the light in the room was sufficient for the 

 animal itself to be seen, the illumination in the eye was more obscure, and ap- 

 peared to arise from the external surface of the iris. 



As the difficulties which occurred in making observations on the illuminated 

 state of the eye in the living animal were so great, an attempt was made to re- 

 peat, as nearly as possible, the experiments after death. In doing so it was found, 

 that a strong light thrown on the cornea illuminated the iris, as it had done in the 

 living eye; but when the cornea was removed, this illumination disappeared. The 

 iris was then dissected off, and the lucid tapetum completely exposed to view; the 

 reflexion from which was extremely bright; the retina proving no obstruction to 

 the rays of light, but appearing equally transparent with the vitreous humour and 

 crystalline lens. 



From these experiments it appeared evident that no light is generated in the eye; 

 the illumination being wholly produced by the concave bright-coloured sur- 

 face of the tapetum, collecting the rays of the external light, concentrated by the 

 cornea and crystalline lens, and reflecting them through the pupil. When the 

 iris is completely open, the degree of brilliancy is the greatest; but, when the iris 

 is partly contracted, which it always is when the external light is increased, then 

 the illumination is more obscure, and appears to come from the iris; a part of the 

 light reflected from the tapetum being thrown back, by the concave surface of the. 

 cornea, on the anterior surface of the iris, giving it a bright shining appearance. 

 The influence which the will of the animal has over this luminous appearance 



