642 PHYSIOLOGY 



over which no variation in colour can be perceived. By passing 

 the rays forming a spectrum through a similar prism placed in the 

 reverse direction, all these rays can be recomposed to form white 

 light. The sensation of white light is therefore due to the simul- 

 taneous incidence on the retina of all the rays of the spectrum. That 

 we have no suspicion of the existence of these rays when we experi- 

 ence a sensation of white shows that our eye does not possess any 

 resolving or analysing apparatus such as exists in the internal ear 

 for the compound wave of sound. 



Any part of the spectrum, or any coloured object, may be char- 

 acterised in three different ways : 



(1) LUMINOSITY. The luminosity of different parts of the 

 spectrum varies, being greatest in the yellow for the light-adapted 

 eye. We could, however, match the luminosity of the red of one 

 spectrum with that of the yellow of a second spectrum by increasing 

 the intensity of the beam of light used to produce the first spectrum. 



(2) SHADE OR COLOUR. It has been reckoned by Konig that 

 in the spectrum we can distinguish 165 different shades of colour. 

 Edridge- Green has shown that when a normal observer screens off 

 the rest of the spectrum until the part left appears monochromatic, 

 and repeats this operation through the whole length of the spectrum, 

 he will mark off not more than eighteen to twenty-seven ' mono- 

 chromatic patches.' There are certain colours which can be appre- 

 ciated by the eye which are not present in the spectrum, such as the 

 varying shades of purple. 



(3) SATURATION. When we look at a coloured surface, e.g. red, our 

 eye is stimulated partly by the white light which is reflected in toto from 

 the surface, partly by the red rays which are specifically reflected and 

 give the colour of the object. According as these red rays are free 

 from mixture with white rays, so their saturation is said to increase. 

 The degree of saturation of any colour can be determined by regarding 

 it through a spectroscope. A completely saturated red would give 

 only rays at the red end of the spectrum. We can, however, 

 speak not only of a psychical but of a physiological saturation. 

 According to the condition of the retina and nature of the stimuli to 

 which it has been previously exposed, so does the saturation of any 

 given colour vary. 



It might at first be thought that the retina could respond with a 

 simple sensation to a stimulus by any part of the spectrum, a low 

 number of ether vibrations per second producing a sensation of red, 

 a number rather higher a sensation of orange ; so that there might be 

 as many simple colour-sensations as we can appreciate different 

 shades in the spectrum. But a simple analysis of our own sensations 

 seems to show that some of the spectral colours, although simple in 



