ERRORS OF APPRECIATION 58l 



condition frequently affects the red end of the spectrum, but it may be found 

 in other parts. The principal effects to be noted are : (1) an abnormal type 

 of luminosity curve, as examined by means of the flicker method ; (2) the 

 requirement of different amounts of the primary colours in order to match 

 a given spectral colour, as compared with individuals with normal vision ; 

 (3) inability to recognise the normal number of different hues in the part 

 of the spectrum affected, as shown by the spectroscope test ; (4) shortening 

 of the spectrum, if either of the ends of the spectrum is affected. The 

 condition would appear to be directly traceable to abnormality in the colour 

 receiving apparatus of which the cones form an important part. These 

 cases therefore show quite distinct features which at once differentiate them 

 from those of class 2. For in that class, so far as can be ascertained, the peri- 

 pheral receiving apparatus is normal, and the error lies with the higher 

 centres in which the resulting nerve impulses are interpreted. A typical 

 example of a case belonging to class 3 will now be described, namely, that 

 in which there is shortening at the red end of the spectrum. The flicker 

 test shows that the luminosity curve for different parts of the spectrum 

 has its maximum in the green, instead of in the yellow. Further, the curve 

 does not extend so far into the red, or show such high values in the orange 

 as the normal curve. This curve therefore explains the apparent shortening 

 of the spectrum. Colour mixture experiments show that the patient can 

 match mixtures of green and blue with white light. When required to match 

 yellow, he uses an excessive amount of red, and correspondingly less 

 "green than the normal sighted ; this test again shows the deficiency of 

 the red sensation. Tested by the spectroscope it is found that, beside the 

 shortening at the red end of the spectrum, there is also an inability to distin- 

 guish the normal number of hues at the green and yellow. This effect is 

 readily explained, because the difference between green and yellow shades 

 largely depends on the varying extent to which they stimulate the red 

 sensation. When the red sensation is absent, it is clear that the differen- 

 tiation of greens and yellows must suffer. 



EFFECT OF INTENSITY ON COLOUR VISION. It is well known 

 that there is a certain range of intensity over which the appreciation of 

 colour is a maximum, and that at high and low intensities appreciation 

 is diminished. Thus at low intensity the spectrum will appear shortened 

 at both red and violet ends, and with the spectroscope test perhaps 10 

 monochromatic areas will be mapped out instead of the normal 20 to 30. 

 At high intensity on the other hand, the spectrum appears to extend further 

 than usual at both red and violet ends, but again il is found that the number- 

 of apparently monochromatic areas is considerably reduced. In the one 

 case it would seem that the impulses received by the brain from the cone 

 mechanism .are so feeble that appreciation is diminished, and in the second 

 that a powerful colour stimulus arouses all three sensations indifferently and 

 therefore makes the differentiation of colour difficult. 



PERIPHERAL VISION. Experiments with the perimeter show that 

 there is under ordinary circumstances a reduced appreciation of colour in 



