PHYSIOLOGICAL 251 



like brilliant rubies. In some of the moths the colour is red, in 

 others golden yellow. Thus our question comes to be: What is 

 there in common between the cat's eyes and the rhinoceros beetle's 

 eyes that both should "shine in the dark"? 



The answer is that both have a reflecting layer in the back of 

 the eye. This reflecting layer — called the tapetum — acts like a 

 concave mirror, and sends out again the scanty rays of light which 

 have already entered through the lens. As the light-rays pass out 

 again through the lens they are concentrated, and thus appear 

 more brilliant than the original source of illumination. In a pocket 

 electric torch the incandescent filament gives of itself no great 

 light, but when passing through the lens the light is concentrated 

 and strengthened. 



In the eyes of the cat and the beetles the light comes, to begin 

 with, not from within, as in the electric torch, but from without, 

 from the lamp. In the cat the reflecting mirror consists of flat cells 

 filled with microscopically minute crystal-like spangles; in the 

 moth the mirror is formed by a network of air-tubes similar to 

 those which take air to every hole and corner of the insect's body. 

 It is interesting that these reflecting mirrors should be made in two 

 quite different ways. 



What can be the use of having eyes that "shine in the dark"? 

 It is not likely that it can be of use to reveal the cat's presence! 

 The answer is that if it were quite pitch dark neither cat nor 

 death's-head moth nor rhinoceros beetle could see at all; but it is 

 very seldom so dark as all that — there is usually some faint illumi- 

 nation, even at dead of night. 



So the ingenious suggestion — it is only a suggestion — has been 

 made that the reflecting of the faint rays of light from the back of 

 the eye forwards again gives the sensitive cells of the eye a "second 

 impression", a second chance to see something. It is a very interesting 

 question. 



The Biology of Tears. — There is an interesting desert lizard 

 in Mexico and California that goes by the name of Horned Toad or 

 Phrynosome. It is a non-aggressive creature, asking only to be left 

 alone, but it becomes greatly excited when it is teased. It is said 

 to shed tears of blood, and this is in a remarkable way the case. 

 When it is much perturbed, there is a rush of blood to the head, 

 and the eyelids become so much congested that they swell to twice 

 or thrice their ordinary size, and from below the upper one there 

 issues a fine jet of blood ! There is a superficial haemorrhage, occurring 

 under very unusual circumstances, and it is such an expensive 

 mode of weeping that we cannot wonder at it being unique. It 

 corresponds in part to a man's eyes becoming blood-shot in a rage, 

 and it throws from a distance some light on weeping. 



Just as with laughter, so in regard to tears, we must not think 



