34 The Perception of Colour. 



yet there is reason to suppose that birds at least have a well- 

 developed colour sense. There had long ago been observed 

 in the rods and cones of the retinas of these animals spherical 

 fatty drops of red and yellow colour, which have been sup- 

 posed by physiologists to be of importance in colour percep- 

 tion, but they differ from the retinal purple in that light has 

 not much effect in bleaching them. An investigation of 

 their nature and properties by Dr. Capranica XAnnales 

 d' Oculistique, Ixxviii., p. 144, 1877) has revealed, however, 

 that as regards solubility and reactions the colouring matter 

 contained in these globules agrees completely with that in 

 the pigment layer of the frog's retina, and that the difference 

 between the red and yellow is only one of concentration. 

 When dissolved in alcohol, chloroform, or sulphuret of carbon, 

 this pigment is decolorised by the action of light, the diffe- 

 rent forms of monochromatic light acting on it as on retina- 

 purple, with which it has therefore the closest affinities. 

 The photo-chemical sensibility, according to Capranica, 

 depends on the amount of fatty matter associated with it. 

 These isolated coloured globules may therefore be presumed 

 to play the same part as the more diffused colour in the 

 retina of the mammalia. 



Enough has been said, I think, to make it at least highly 

 probable that the perception of colours is essentially con- 

 nected with photo-chemical processes, and the admission of 

 this interpretation has the further advantage that it brings 

 this function into closer analogy with other special senses, 

 the optic fibres being stimulated by particles of chemical 

 substances just as the olfactory and gustatory nerves are by 

 particles of odorous and sapid substances, and the auditory 

 nerve terminations by mechanical pressure or the impact of 

 the minute bodies known as otoliths. 



In addition to the references given in this and the previous 

 communications, I may state that the data on which the 

 argument in this paper is based have been obtained mainly 

 from the following authorities : — 



(1.) A review of the literature on retina-purple in the 

 American Journal of the Medical Sciences, July, 

 1878. 



(2.) Wilhelm Schoen. Die ■ Lehre vom Gesichtsfelde und 

 seinen Anomalien, 1874. 



(3.) Hermann. Human Physiology (English translation), 

 1875. 



(4.) Wilhelm Wundt. Lehrhuch der Physiologic, 1868. 



