336 
im. W. POLE ON COLOLErBLINDNESS. 
much in severity, and inclines strongly to the opinion that the number of instances 
where the vision is perfectly dichromic, i. e. where the true sensation of red is altogether 
imperceptible, are very few. 
It is with diffidence I would dissent from the opinion of such an authority ; but as 
this is a point confessedly obscure, I do not hesitate to record my own impression that 
the cases of perfect dichromatism are much more common than Dr. "Wilsox supposes, 
and that, in fact, the majority of cases where well-defined symptoms occur, may be, and 
probably are, purely dichromic. 
22. The opinion in favour of the varied character of the disorder is doubtless suggested 
by the nature of the testimony ; for on examining it closely we discover differences in 
the descriptions which would seem to indicate considerable variety between the sensa- 
tions of different parties. But granting, without hesitation, that each individual may 
have faithfully described what he thinks he sees, there are some considerations which 
must ever render it necessary to use great caution in interpreting the testimony of the 
colour-blind, and which, if not allowed for, may lead to the deduction of very erroneous 
inferences from what they say. 
23. In the first place they must be very liable to associate, almost indissolubly, the 
true normal name of a colour with the sensation it conveys to their minds, whatever that 
se)isation may he ; and they may therefore easily be led to speak of that colour' as if they 
saw it like other people, although the sensation they refer to may be really of quite a 
different nature to that which the name implies. A colour-blind person will be espe- 
cially loth to believe that certain colours, which he hears about and talks about every 
hour of the day, can be invisible to him. Objects of these hues will probably present 
to his mind some ideas of colour (though not the true ones), and he may naturally 
imagine therefore that he does see them, and may give his description accordingly. 
And this source of error is very much enhanced by the fact that it is not an easy 
matter always to refer different tones of any one colour to the same colour-sensation ; so 
that a modification of tone, if considerable, may be easily supposed to be a different 
colour. The sensations excited by the higher, the middle, and the lower dhisions of 
any one of the “ gammes chromatiques” are so different from each other, that it requires 
no small degree of reflection and judgment to be able to identify the difference as due 
to black or white only, independent of any other colouring agent. 
I believe this difficulty is also felt by the normal-eyed, and the popular nomencla- 
ture of colours furnishes illustrations of the fact, different tones of the same colour 
having often different names, and being treated as separate colours. Pink and crimson, 
lilac and violet, are well-acknowledged examples of this ; and a dark shade of orange is 
called brown, which generally passes for a separate colour. Persons not well versed in 
the principles of colour, are often reluctant to admit that these variations of tone are 
really the same hue. Hence we may easily see what a great probability there is that 
the colour-blind may acquire the habit of attaching the names of different colours to 
what are in reality only varieties of the same sensation ; and as this habit dates from 
