574 PHYSIOLOGY 



axes are directed on it, is seen in its true colour as soon as its intensity is suffi- 

 cient for it to be seen at all. Before this takes place it may be seen by the 

 peripheral parts of the retina, but as a colourless spot of light which dis- 

 appears as soon as the gaze is directed on it. This marked difference between 

 the behaviour of the fovea centralis and the more peripheral parts of the 

 letina in the dark-adapted eye has been attributed to the difference we have 

 already studied in the anatomical structure of these parts. The only per- 

 cipient elements found in the fovea centralis are the cones. In the adjacent 

 portion of the retina we find also the rods which increase in relative number 

 as we pass from the central spot towards the periphery. Schultz long ago 

 pointed out that in the retinae of many night animals, such as the owl, the 

 mouse, the cat, the rods are the predominating element, the cones being 

 absent or very few in number. Von Kries has suggested that in all proba- 

 bility the retina is endowed with two kinds of vision. 



(a) Vision by means of rods, which are colour-blind, so that on stimula- 

 tion by any rays of the retina a sensation of white or grey is produced. 

 The rods are chiefly excited by the more refrangible rays of the spectrum, 

 being totally unaffected by the red rays. They show great power of adapta- 

 tion. This form of rod vision may be connected with the visual purple. In 

 the dark-adapted eye this pigment is found pervading the whole of the outer 

 limbs of the rods ; it rapidly fades on exposure of the eye to light, so that it 

 must be absent in the light-adapted eye. On examining its absorption 

 spectrum we find that its absorptive power is greatest for the rays in the green 

 part of the spectrum, and that it allows the red rays to pass almost without 

 absorption, i.e. it absorbs just those rays which experiment shows us have 

 the greatest effect in producing a sensation of light in the dark-adapted eye. 



(6) The cones, on this view, would represent a more highly differentiated 

 apparatus of vision. They alone are present in that part of the retina 

 which we use exclusively for distinguishing the finer details of surrounding 

 objects ; they are sensitive to all colours, and when stimulated by all the 

 rays of the spectrum simultaneously give rise to a sensation of white light. 

 Their sensitiveness to illumination is, however, inferior to that of the rod 

 apparatus. According to this theory, therefore, whereas in a dim light we 

 determine the position of surrounding objects and differences in their 

 luminosity by means of the rods, the greater part of our visual impressions, 

 including all that we obtain by daylight and our knowledge of the finer 

 visual qualities of things, are brought to us by the intermediation of the 

 cones. 



COLOUR VISION 



If a ray of white light be passed through a prism it is widened out into a 

 bright coloured band or spectrum, the red rays, which are least refrangible, 

 being at one end, and the blue rays at the other. It is usual to divide the 

 colours of the spectrum into seven red, orange, yellow, green, blue, indigo, 

 violet ; but the division is an arbitrary one, and the colours shade into one 

 another so gradually that no two observers would agree exactly on the 

 limits between them. The difference of wave-length necessary to give a dis- 



