TJie Discrimination of Colour. 



117 



chromatic region. I have magnified the image of this region with eyepieces 

 of different power, making a corresponding increase in the light to make up 

 for the loss of luminosity caused by the magnification. I have also obstructed 

 the central portion of the monochromatic region with a screen, and the 

 remaining portions have still appeared monochromatic. The most conclusive 

 experiment, however, is the examination of the monochromatic region with 

 an achromatic double image prism, the intensity of the source of light being 

 increased as before. By this method two rectangular monochromatic fields 

 are seen, and can be arranged so that they are side by side and just touch. 

 The portion belonging to the red side of the spectrum of one can be made to 

 touch the portion belonging to the violet side of the spectrum of the other. 

 This position is therefore most favourable for the detection of any difference, 

 and yet I cannot detect any, neither can any other observer to whom I have 

 shown the experiment. The experiment can be observed objectively in the 

 following manner: An arc light being used for the illuminating source, 

 a pure spectrum is obtained ; a portion of this spectrum, forming a mono- 

 chromatic region, is allowed to pass through an adjustable slit. Two images 

 of this monochromatic region can be thrown with the aid of a double image 

 prism upon a screen and made of any required size. The varying size of the 

 monochromatic regions with different persons can by this means be demon- 

 strated to a large number. 



Another point which I found with Lord Eayleigh's apparatus is the 

 difficulty of obtaining both fields of similar intensity. The slightest 

 movement of the eye also causes an alteration in the number and kind 

 of rays which enter the eye. When two fields are of unequal intensity 

 the physiological effect of contrast is evoked, which causes an erroneous 

 judgment of the colours under observation. In fact, weak orange light may, 

 by contrast with bright red light, appear green. In making experiments on 

 the discrimination of colour, the rays of light from the two regions to be 

 compared should strike the eye at as nearly as possible the same angle ; the 

 fixation point of the eye should be in the centre between the two regions, so 

 that one region may not be more influenced by the pigment of the yellow 

 spot, or the blood in the retina, than the other ; and equal amounts of light 

 from each region should enter the eye. 



The importance of the irregularly dispersed light in association with 

 contrast in dealing with questions of colour has been overlooked by many 

 physicists, as several instruments have been constructed for the investigation 

 of colour and colour- vision, which are defective on this ground. It was this 

 irregularly dispersed light, as shown by Helmholtz,* which caused the apparent 

 * ' PoggendorfFs Annalen,' 1852, No. 8. 



