VISUAL SENSATIONS 575 



tinct difference in colour varies according to the part of the spectrum which 

 is under observation. In the middle of the spectrum it is between O7 /ULJUL and 

 2O /UL/UL. At the red end a difference of 4-7 /xyu is required to evoke a new 

 quality of sensation, and at the extreme end, both red and violet, there is a 

 section of the spectrum 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 simultaneous incidence on the 

 retina of all the rays of the spectrum. That we have no suspicion of the 

 existence of these rays when we experience 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 characterised 

 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 ' monochromatic patches.' There are certain colours 

 which can be appreciated 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 spectro- 

 scope. A completely saturated red would give only rays at the red end of the 

 spectrum. We can, however, speak not only of a physical but of a physio- 

 logical 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-sensa- 

 tions 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 so far as the stimuli are concerned, are compound so far as 

 the sensation is concerned. Thus most people would say at once that orange 



