RELATIONSHIP BETWEEN STIMULUS AND SENSATION 557 



rod- visual purple apparatus is responsible for the appreciation of the light 

 of low intensity ; it is therefore found in all parts of the retina other than 

 the fovea centralis from which rods are absent. The effect of time of stimulus 

 will be considered shortly. 



INTENSITY THRESHOLD FOR COLOUR 



If a spectrum of low intensity which appears colourless to the eye be 

 gradually increased in brightness, a point will be reached at which the colours 

 begin to be recognisable, first yellow and green, then blue and lastly red and 

 violet. If the intensity at which the colour just vanishes is measured, 

 the curve obtained is similar to that shown in Fig. 287. As the 



\ 



\ 



70 



60 



SO 



20 



S 10 IS 20 25 30 SS 40 45 SO 55 



Fro. 287. 'Extinction of colour' curve. Abscissae = wavelengths; ordinates = inten- 

 sity in candle feet when colour just vanishes. (ABNEY.) 



intensity is increased, the point of maximum luminosity gradually 

 shifts from the green to the yellow. As a coloured object is 

 gradually increased in intensity, it is first seen without colour, but 

 after an interval the colour also is recognised ; this is called the photo- 

 chromatic interval. It follows from what we have said that the interval 

 is greatest for colours of short wavelength (blue) and least for long (red) 

 (the Purkinje phenomenon). The thresholds for light and colour differ in 

 another important respect, namely that whereas that of light varies with the 

 degree of dark adaptation, that of colour is found by experiment to be nearly 

 constant. With regard to the effect of area of light source, it is found that 

 the same type of relationship exists in the case of colour as for light, 

 namely, that as the area is decreased, so the intensity must be correspond- 

 ingly increased. The area and intensity, multiplied together, do not however 

 equal a constant as they do in the case of light. The appreciation of colour 

 is associated with the cones and is therefore most highly developed at 



