RECEPTOR ORGANS 525 



wave lengths 770 and 396 /x/z about. Red and violet would be distinguished 

 first, next green between them would be added, finally yellow, and blue. 

 Correspondingly, a common form of colour blindness is the tri-chromatic, where 

 red, green, and violet are the only colours perceived. Yellow is called red-green, 

 and blue, green-yellow. 



A further important point established by this investigator is that, contrary to 

 what a casual examination of the spectrum might lead one to suppose, there is not 

 an infinite series of gradations of colour along the spectrum, but that it can be 

 divided up into a number of patches, each of these patches being of a uniform 

 colour. Thus the eye is not capable of appreciating an indefinite number of 

 spectral colours. The fact can be shown by the use of a spectrometer with 

 adjustable shutters in the ocular. When any part of the spectrum is thus 

 isolated, it is found that a certain breadth can be found which appears to be all 

 of the same colour. Thus the whole spectrum is divided up, by normal sighted 

 people, into some sixteen to twenty monochromatic areas. 



Edridge-Green has brought out methods of testing colour vision on the basis 

 of the above facts, together with other considerations. These methods are now 

 being accepted as the only reliable ones. 



The existence of colour vision in the animals lower than man is obviously 

 a difficult matter to decide. Orbeli (1909), in his work on conditioned reflexes, 

 found the dog unable to form such reflexes to colour alone, merely to differences 

 of luminosity. Later observers found that, with great difficulty, colour can be 

 used in this way. The colour sense must be very rudimentary in the dog. 

 Frohlich (1913) thinks that the difference between the electrical changes to red 

 and to blue in the Cephalopod indicates' colour vision, but since differences in 

 intensity of white are also associated with differences of rate of rhythm, the 

 only evidence is the quantitative one of the rapid diminution in comparative 

 effect as the intensity of the stimulus is increased. The apparent fondness of 

 certain birds for brilliant colours, and, in fact, the general evolution of colour in 

 flowers and butterflies and so on, suggests some sort of colour sense in these 

 animals. According to Frisch (1914), a sense of colour is shown by fishes. 



The numerous phenomena connected with positive and negative after-images 

 are beyond the scope of this book. One fact should, however, be noticed, namely, 

 that certain combinations of spectral colours give what appear to the eye to be 

 colours as pure as the spectral colours themselves, but of a different wave length 

 from those of which they are composed. For instance, red and green give 

 a yellow, which is indistinguishable from spectral yellow. This fact is not easy 

 to explain. Hartridge (1912) gives reasons for holding that the effect may be 

 merely physical, so that the yellow-perceiving mechanism may really be excited 

 by the mixture of red and green. (See also Edridge-Green, 1915.) 



Mention should also be made of the new apparatus of A. W. Porter, which 

 is the most perfect yet devised for the investigation of colour mixing, after- 

 images and other colour phenomena. This instrument has been shown at the 

 Physiological Society's Meeting, but the description has not yet been published. 



Mosaic Vision. The compound eye of the insect and crustacean is a highly 

 developed organ and is usually considered to act as a series of tubes, with opaque 

 walls, by which that ray only which is a continuation of the axis of the tube 

 arrives at the receptor mechanism. In this way an image is formed. The 

 explanation of the elaborate structures present, some of which appear to be 

 refractile, is uncertain. The monograph by Exner (1891) may be consulted. 



RECEPTORS FOR SOUND 



In order that the rhythmic vibrations of bodies, which are the material basis 

 of what we ourselves call sound, may excite the ends of nerve fibres, it would 

 seem that the most natural way would be to make use of the principle of resonance, 

 which has been described on page 88 above. 



If we have a series of strings in regular order as regards their period of 

 vibration, sound waves of a particular period will affect one of these strings only 



