CRANIAL NERVES AND SENSE-ORGANS 247 



ceived by an artificial arrangement, and a peculiar sensation 

 of the solid is developed in us, which the monocular person 

 seems not to be acquainted with " (Bonders). 1 



Light-perception, the appreciation of various intensities of 

 illumination, has been very inadequately investigated from the 

 physical standpoint. In the rods and cones of all vertebrate 

 retinas (except amphioxus) is contained a peculiar pigment, 

 the visual purple, which is acted upon by light, and in the 

 absence of any other explanation appears to be the seat of a 

 photo-chemical process in the retina. 



There is a similar uncertainty as to the physiological ex- 

 planation of colour-sense which has been provisionally attributed 

 to the cones. In justification of this theory the occurrence of 

 pigmented oil globules in the cones of birds is pointed out ; 

 but on the other hand many nocturnal creatures have either no 

 cones or but an insignificant number, and yet have undoubted 

 colour-perception. 



It is well known that man and animals have colour-vision, 

 but the perceptions cannot be identical (Schleich), for most 

 animals can appreciate the ultra-violet rays, so that for them 

 the spectrum not only of white but of every other colour must 

 extend much further than in man, and result in a totally dif- 

 ferent series of colour-sensations. 



Schleich divides animals into two classes, light-loving (i.e., 

 with a preference for blue rays mammals and some birds), and 

 dark-loving (i.e., with a preference for red rays certain birds). 



Lubbock has shown experimentally that even Invertebrates 

 (insects and crustaceans) can distinguish colours. He found, 

 for instance, that bees evince a preference for blue and in de- 

 scending order white, yellow, red, green and orange. Lubbock 

 and Forel have attributed to ants appreciation of the ultra-violet 

 rays ; man cannot perceive them. As evidence that they can 

 actually see these rays is the fact that varnishing the eyes makes 

 no difference to them. 



Finally, just as certain apes have special predilections for 

 particular colours (e.g., the Capuchin ape for green), so have 

 individual men and races comparable tastes in colours. Dr. 



1 Bonders, " Accommodation and Refraction of the Eye," New Sydenham 

 Soc.. 1864, p. 161. 



