COLOUR-BLINDNESS. 505 
sensible difference in the degree of heat conveyed to the point 
of the finger.* 
The fact that a difference of tint is recognized, although the 
eye of the colour-blind person does not appreciate any dif- 
ference of colour, as red and green, when compared together, 
and that every one is educated to call things by certain names, 
whether he understands the meaning or not, may help to 
explain the slowness with which this defect is discovered; 
and again, that the report of every case is rendered hopelessly 
imperfect from the impossibility of subjecting the eye to the 
test of colour. 
In the “Philosophical Transactions for 1859,” Mr. W. Pole, 
a well-known civil engineer, thus describes his own case : — “ I 
was about eight years old when the mistaking a piece of red cloth 
for a green leaf betrayed the existence of some peculiarity in 
my ideas of colours ; and as I grew older continued errors of a 
similar kind led my friends to suspect that my eyesight was 
defective ; but I myself could not comprehend this, insisting 
that I saw colours clearly enough, and only mistook their 
names. I was articled to a civil engineer, and had to go 
through many years of practice in making drawings of the 
kind connected with this profession. These are frequently 
coloured, and I recollect often being obliged to ask in copying 
a drawing what colours I ought to use ; but these difficulties 
left no permanent impression, and up to a mature age I had 
no suspicion that my vision was different from that of other 
people. I frequently made mistakes, and noticed many cir- 
cumstances in regard to colours which temporarily perplexed 
me. I recollect in particular having wondered why the beau- 
tiful rose light of sunset on the Alps, which threw my friends 
into raptures, seemed all a delusion to me. I still, however, 
adhered to my first opinion, that I was only at fault in regard 
to the names of colours, and not as to the ideas of them ; and 
this opinion was strengthened by observing that the persons 
who were attempting to point out my mistakes often disputed 
among themselves as to what certain hues of colour ought to be 
called.” At length Mr. Pole when about thirty years of age 
committed a glaring blunder, and this circumstance led him 
to make an investigation of his case, which ended in his 
discovering that he was colour-blind. 
All who have investigated the subject of colour-blindness 
* Professor Tyndall enters fully into all these matters in his work on 
“ Heat as a Mode of Motion” (reviewed in the present number). The eighth 
chapter treats of the relations of light, heat, and colour, and will be found 
deeply interesting to those who have read this article, and desire further 
information on the subject. — E d. 
