Curiosities of Science. 53 



incident rays of light, and thus produce the shining ; and this 

 appeared more probable as the light of the eye now seemed to 

 emanate from a single spot. Having taken away the vitreous 

 humour, Dr. Esser observed that the entire want of the pigment 

 in the hinder part of the choroid coat, where the optic nerve 

 enters, formed a greenish, silver-coloured, changeable oblong 

 spot, which was not symmetrical, but surrounded the optic 

 nerve so that the greater part was above and only the smaller 

 part below it ; wherefore the greater part lay beyond the axis 

 of vision. It is this spot, therefore, that produces the reflection 

 of the incident rays of light, and beyond all doubt, according 

 to its tint, contributes to the different colouring of the light. 

 It may be as well to explain that the interior of the eye is 

 coated with a black pigment, which has the same effect as the 

 black colour given to the inner surface of optical instruments : 

 it absorbs any rays of light that may be reflected within the eye, 

 and prevents them from being thrown again upon the retina 

 so as to interfere with the distinctness of the images formed 

 upon it. The retina is very transparent ; and if the surface be- 

 hind it, instead of being of a dark colour, were capable of re- 

 flecting light, the luminous rays which had already acted on 

 the retina would be reflected back again through it, and not 

 only dazzle from excess of light, but also confuse and render 

 indistinct the images formed on the retina. Now in the case 

 of the cat this black pigment, or a portion of it, is wanting ; and 

 those parts of the eye from which it is absent, having either a 

 white or a metallic lustre, are called the tapetum. The small- 

 est portion of light entering from it is reflected as by a concave 

 mirror ; and hence it is that the eyes of animals provided with 

 this structure are luminous in a very faint light. 



These experiments and observations show that the shining 

 of the eyes of the cat does not arise from a phosphoric light, 

 but only from a reflected light ; that consequently it is not an 

 effect of the will of the animal, or of violent passions ; that 

 their shining does not appear in absolute darkness ; and that 

 it cannot enable the animal to move securely in the dark. 



It has been proved by experiment that there exists a set of 

 rays of light of far higher refrangibility than those seen in the 

 ordinary Newtonian spectrum. Mr. Hunt considers it probable 

 that these highly refrangible rays, although under ordinary 

 circumstances invisible to the human eye, may be adapted to 

 produce the necessary degree of excitement upon which vision 

 depends in the optic nerves of the night-roaming animals. The 

 bat, the owl, and the cat may see in the gloom of the night 

 by the aid of rays which are invisible to, or inactive on, the 

 eyes of man or those animals which require the light of day 

 for perfect vision. 



