1892 - 93 .] Dr William Pole on Colour-Blindness. 121 
Thus, out of 31 persons observed — 
24 may be called normal. 
5 differ considerably from them, in having either much 
increased susceptibility for the red or much diminished 
susceptibility for green. 
2 have abnormal susceptibility in the opposite direction. 
The author adds : — “ Mr Balfour requires only half as much red 
as myself in order to turn a given amount of green into yellow ; on 
the other hand, Mr Hart requires much more red than I do, in the 
ratio of about 2*6 to 1.” 
W. 
Experiments by Von Kries and Frey^ 1881. 
These observers, after referring to the work of Maxwell and 
Muller, undertook experiments partly to obtain the results of a 
series of mixtures of spectral colours, partly to learn something 
more exact about the individual differences of observers, to which 
the phenomena of colour-blindness had given a special interest. 
Their first series of experiments was the direct determination of 
complementary colours by two different observers. Their next was 
the mixing of red and green to make yellow ; and the next for 
mixtures of green and violet. The observations showed that the 
results varied with the two observers from 20 to 50 per cent. 
They thought that the most obvious explanation was the 
assumption of a difference in excitability of the sensitive fibres, 
or absorption by the yellow pigment of the retina. 
X. 
Experiments by Bonders on Variations of Normal Vision. 
He tested 60 persons, all, generally speaking, normal eyed, by 
Tuixing a red at the lithium line (670) with a green at the thallium 
line (535) in such quantities as to form the yellow of the sodium 
line (590). 
Fifty-six persons ranged from 31 green -f 69 red to 23 green + 77 
red. 
