i88 5 .] 
Testing for Colour-blindness. 415 
yielding to the wish of a son for a sea-faring life. The inability 
of a boy to distinguish colours commonly in use should at 
once settle the question of his taking to the sea at all. 
Colour-blindness is, to some extent, aggravated by age, 
just as is the case with certain other defeats of vision. It 
apparently may be developed in after life, and as the near 
point of vision recedes. The Academy at this moment 
furnishes abundant examples, and which seem to justify the 
criticism that £< yards of garish canvasses wanting in ideas, 
composition, drawing, and colour ” cover its walls. To 
what other cause, if not to a great deterioration of the 
colour-sense, or to colour-blindness, can this great falling 
off in the works of artists be attributed ? 
As to the physical nature of colour-blindness, no entirely 
satisfactory explanation has been offered. Hering advanced 
a plausible theory that colour is the mental perception of 
the changes taking place in the visual substance, which, 
under the influence of light, is constantly undergoing a double 
process of disintegration and reparation. Perception of 
white light being coincident with disintegration ; of black- 
ness with re-integration ; the degrees of white and black de- 
pending on the activity of the processes of disintegration and 
repair. But of the many theories promulgated, that known 
as the Young-Helmholtz is regarded as the most acceptable. 
According to this theory, the retina has three kinds of colour- 
perceiving elements, the stimulation of which gives respec- 
tively the sensation of red, green, and violet. White light 
excites all the elements equally, but if monochromatic or 
homogeneous light be received upon the retina, then each of 
the three kinds of fibres are simultaneously stimulated, and 
with an intensity that varies with the length of the waves. 
Thus, red light, which has waves of the greatest length, 
stimulates the red elements strongly, the green more feebly, 
and the violet only slightly, consequently the sensation ex- 
perienced is red. Green, which has waves of intermediate 
length, stimulates the red and the violet feebly, and the 
green elements strongly, and the sensation perceived is 
green, and so with the violet, which has waves of the shortest 
length, and which aCts as a powerful stimulus to the violet 
elements of the retina, and scarcely affeCts the green and red 
elements at all. 
From what I have stated it may be understood that there 
are three kinds of colour-blindness corresponding to the 
three colour-perceiving elements : red blindness, green blind- 
ness, and blue or violet blindness. Of the several layers of 
which the retina is made up, it is the layer of rods and cones 
