426 CLASS MAMMALIA. 



proved, that when the external light is wholly excluded, 

 none can be seen in the Cat's eye. The truth of the results 

 was readily ascertained ; it therefore only remained to 

 determine, whether the external light is of itself capable 

 of producing so great a degree of illumination : this was 

 attended with difficulty ; for, when the apartment is dark- 

 ened, and nothing but the light from the Cat's eye seen, the 

 animal, by change of posture, may immediately deprive the 

 observer of all light from that source. 



This was found to be the case, whether the common Cat, 

 the Tiger, or the Hyaena, was the subject of the experiment. 

 On the other hand, when the light in the room is sufficient 

 for the animal to be seen, the light in the eye is obscured, 

 and appears to arise from the iris. 



As these difficulties occurred in making experiments on 

 the living eye, others were made immediately after death, 

 and it was found, that a strong light thrown upon the cor- 

 nea illuminated the eye, as in the living animal, but when 

 the cornea was removed, the illumination disappeared. 

 The iris was then dissected off, and the tapetum lucidum 

 completely exposed to view, the reflection from which was 

 extremely bright ; the retina proving no obstruction to the 

 rays of light, but appearing equally transparent with the 

 lens and vitreous humour. 



These experiments prove, that no light is generated in 

 the eye, the illumination being wholly produced by the ex- 

 ternal rays of light collected in the concave bright-coloured 

 surface of the tapetum, after having been concentrated by 

 the cornea and crystalline lens, and then reflected through 

 the pupil. When the iris is completely open, the light is 

 the greatest, but when the iris is contracted, the illumina- 

 tion is more obscure. 



The influence, if any, the animal has over this luminous 

 appearance, depends on the action of the iris : when it is 

 shut, no light is seen; when the animal is alarmed, the eye 

 glares, the iris being opened. 



