500 
POPULAR SCIENCE REVIEW. 
indicates the absence of determinate sensation, depending 
upon some undiscovered structure or organic arrangement, 
which' forms one-third of the apparatus by which we receive 
sensations of colour. 
“ Suppose the absent structure to be that which is brought 
most into play when red light falls on our eyes, then to the 
colour-blincl red light will be visible only so far as it affects the 
other two sensations, say of blue and green. It will, there- 
fore, appear to them much less bright than to us, and will 
excite a sensation not distinguishable from that of a bluish- 
green light.” 
That is 'to say, the normal eye reduces its colour-sensations 
to three, and analyzes white light into three coloured elements, 
one of which is red ; and that the colour-blind eye, on the 
other hand, reduces its colour-sensations to two, and analyzes 
white light into two elements, neither of which is red ; for 
colour-blindness takes its character more from its non- 
recognition of red than its positive recognition of yellow and 
violet. An essential distinction which can thus be drawn 
between perfect vision and colour-blindness has induced Sir 
J. Herschell to adopt the term dichromatic (cognizant only of 
two colours) to characterize the colour-blind.* We shall now 
examine how far the withdrawal of the red ray affects other 
colours. In the first place, all the light tints, as well as the 
dark tints, are liable to be mistaken for each other. The 
orang-e is no longer red and yellow, but dark yellow ; the 
yellow is purer, the green distinct, the blue purer, and the 
indigo and violet no longer red and blue, but blue mixed with 
more or less black, the violet being the darkest, as containing 
least blue in proportion to red, while the red part itself, 
though not seen as a colour, is not perfectly black. The red 
is generally seen as grey, or neutral tint; the orange as a 
dingy yellow ; the blue as a dirty indigo, and the violet as a 
pale blue, mixed with black and grey. 
In the “ Philosophical Magazine” for 1857 and 1862 will be 
found a series of experiments, instituted by Professor Maxwell, 
to test the accuracy of his own eyes in distinguishing between 
shades of colour ; and his data may be followed by any one 
curious in the same field of inquiry. A large variety of all 
shades and tints of coloured wools may be used for the purpose. 
They should be placed in a mixed heap before the person, who 
must try to arrange and name them, beginning with the 
* Dr. Wilson employs the term/ chromato-pscudopsis (false vision of 
colours ), as it, he says, “ very fairly expresses the general character of the 
affection, which more frecpiently shows itself as an insensibility to certain 
colours, than as a total inability to discern them.” 
