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Proceedings of Boyal Society of Edinhurgli. [sess. 
But this is not all that I want you to do. I especially want to find out 
whether a person can, by using the spectacles repeatedly, always with the red 
glass over the same eye, acquire the power of applying the test without any 
conscious process of reasoning, just as we estimate distances by the eye. Of 
course this is a matter requiring time and trouble, and I have not persuaded 
any other colour-blind person to go through with it, and I only ask you to do 
so because you take an interest, not only in colour-blindness, but in other 
“sensational science,” such as sound, and therefore are able to attend to your 
sensations in a direct manner better than most people who only use their 
sensations as a means of perceiving things. 
With respect to the illustration I made use of concerning the perception of 
distance there is this difference. The distance of a visible object is a matter of 
great importance in every case, and every one has a clear idea of it. But the 
distinction of red and green is of small importance to you, and the necessity of 
it is not always pressing upon you, as in the case of distance. Besides, you 
can never acquire our notion of what is meant by it. 
If I were to wear a pair of spectacles made of Nicol’s prisms, and could turn 
them, or my head, easily round, I should get the power of perceiving the plane 
of polarisation of light, and it is possible that by constantly attending to the 
connection between the appearance of polished bodies and the plane of reflec- 
tion of the light, I might come to perceive unconsciously the inclination of any 
polished surface to the line of sight, so as to have a much more vivid percep- 
tion of the solidity, say of a jet ornament or a polished boot, than I now have. 
In fact, we have a certain degree of this faculty, for the central spot of the 
retina has an imperfect power of analysing polarised light, and the thing seen 
goes by the name of Haidinger’s brushes. When I look at a bit of blue sk}q I 
generally see the sign of the brush, which tells me in what direction the sun 
is ; but as the phenomenon is not a very obtrusive one, I am not surprised that 
many people never observe it, and that it has not become developed into a new 
power of perception. 
If you should be able, by means of the spectacles or otherwise, to assist at 
the transformation of a scientific testing process into a new power of a so-called 
direct perception, you will have conferred a great favour, I do not say on me, 
but on the human mind ; and if you do not succeed, the spectacles will be a 
monument of a defeated curiosity. 
Yours truly, 
T. Clerk-Maxwell. 
I have the spectacles still, and have used them frequently ; and 
although I cannot say I have arrived at the full result contemplated 
by Professor Maxwell, I have abundantly proved the power they 
give of an instantaneous and most positive distinction between hues 
of red and green which appear alike, or nearly so, to the naked eye. 
If, for instance, I look at the three squares in fig. 3 of my paper in 
the quarto Transactions (which to my naked eye appear all alike), 
using the red glass, the red figure is much brightened and the green 
one much darkened ; while with the green glass these effects are 
reversed, the middle figure being unaffected in both cases. 
