THE SENSES 



897 



Even when looked at in reflected light, they are seen by rays that 

 have penetrated a certain way into the substance and have then been 

 reflected ; and, of course, a smaller number of the rays which the 

 body specially absorbs are rejected than of the rays which it readily 

 transmits, for more of the latter than of the former reach any given 

 depth. This is called ' body colour ' ; and such substances have the 

 same colour when seen by reflected and by transmitted light. The 

 colour of haemoglobin is due to the absorption of the violet and many 

 of the yellow and green rays, as is shown by the position of the 

 absorption bands in its spectrum (p. 44). In Fig. 380 the violet rays 

 are represented as being totally absorbed before passing through the 

 substance. Some of the green rays are reflected, some transmitted, 

 some absorbed. The red rays are supposed to be mostly reflected 

 and transmitted, only to a slight extent absorbed. The colour of 

 such a substance, both when looked at and when looked through, 

 would therefore be that due to a mixture of red light with a smaller 

 quantity of green. Then there is another 

 class of substances which owe their colour 

 to selective reflection. Certain rays only 

 are reflected from their surface, and the 

 light transmitted through a thin layer is 

 complementary to the reflected light 

 that is, the reflected and transmitted 

 rays together would make up white light. 

 These bodies have what is called ' surface 

 colour,' and include metals, various aniline 

 dyes, and other substances. 



Comparative. Many invertebrate ani- 

 mals possess rudimentary sense-organs, 

 by means of which they may receive 

 certain luminous impressions. It is true 

 that the mere sensation of light is not in 

 itself sufficient for the exact appreciation 

 of the form and situation of surrounding 

 objects. But even the closure of the eye- 

 lids docs not prevent a person of normal 

 eyesight from distinguishing differences 

 in the intensity of illumination. And it 

 is possible that many of the humbler ani- 

 mals may, through the pigment spots 



which are often called eyes, or perhaps, as in the earthworm, by means 

 of end-organs more generally diffused in the skin, attain to some such 

 dim consciousness of light and shadow as will enable them to avoid 

 an obstacle or an enemy, to seek the sunny side of a boulder or the 

 obscurity of an overhanging ledge of rock. But the indispensable 

 condition of distinct vision is that an image of each part of an object 

 should be formed upon a separate portion of the receiving or sensitive 

 surface. This condition is, to a certain extent, fulfilled by the com- 

 pound eyes of some of the higher invertebrates (insects, e.g.). Here 

 rays from one point of the object pass through one of the funnel- 

 shaped elements of the compound eye, and rays from another point 

 through another. Rays striking obliquely on the facets are stopped 

 by the opaque partitions between them'. In the Cephalopoda we 

 find that this compound type of eye has already been abandoned ; 

 the single system of curved refracting surfaces so characteristic of the 

 vertebrate eye has made its appearance ; and the formation of a 



57 



FIG. 380. DIAGRAM TO SHOW 

 CONNECTION OF BODY 

 COLOUR WITH SELECTIVE 

 ABSORPTION. 



