562 PHYSIOLOGY 



intensity ; thus Helmholtz found it to be a r( Uth, other observers lia\<- 

 obtained even higher fractions. It is interesting to find that tho results are 

 not influenced by the size of the pupil. 



Difference Threshold of Colour 



If the range of colours exhibited in the spectrum be carefully examined, it 

 will be seen that there are certain parts, notably at the red and violet ends, 

 at which the change of colour with wavelength is a very gradual one. At 

 other parts on the contrary the change of hue is very rapid, the yellow region 

 at 5800 A.U. may be given as example. If therefore we determine by experi- 

 ment what difference of wavelength is just perceivable by the eye, we find 

 that it varies with the part of the spectrum under observation. We may 

 therefore conveniently express the difference threshold in different parts 

 of the spectrum in the form of a curve, as in Fig. 289. In persons with normal 

 vision the total number of different hues in the spectrum is calculated to be 

 165. In persons with colour-blindness the number is greatly reduced. 



8-0 



WO 50 500 50 600 50 700 



FIG. 289. Curve showing difference threshold for colour at different parts of the 



spectrum. 



'Abscissae = wavelengths. Ordinates = difference between wavelengths cap- 

 able of being discriminated. (STEINDLER.) 



This fact has been applied by Edridge Green for the detection of colour- 

 blindness ; details of the method will be given later. It should be pointed 

 out that, since in this method the spectrum itself is presented to the 

 observer so that there is a gradual change from one colour to the next, it 

 is the threshold of rate of change of colour that is determined, and not differ- 

 ence threshold of colour. 



Difference Threshold of Saturation 



By saturation is meant the amount of white light which is present with 

 and is therefore diluting a colour. The threshold would appear to be 



