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the same results as I got before. Why I should have 
this tendency to make one column a little larger than the 
other I do not know; possibly it may be some peculiarity of 
vision confined to myself. In the course of my experiments 
I have also noticed the following curious phenomenon, and 
this repeatedly, when working with solutions coloured with 
bichromate of potash, and with ammonio-sulphate of copper ; 
look steadily with one eye, say the right, through the solu- 
tion at a white surface, after the lapse of abour a minute 
suddenly turn the head so as to bring the left eye close over 
the cylinder, then the colour will seem more intense than it 
did with the right ; having looked with the left eye for about 
a minute, bring again the right eye suddenly close over the 
cylinder, and the colour will seem more intense than it did 
with the left, and so on alternately. It would seem as if 
the first impressions of colour on the eye were the stronger, 
and as if there were a gradual and imperceptible decrease in 
intensity — perhaps alterations in the aperture of the pupil 
may contribute to this. Another matter for consideration 
in colorimetry is the nature of the incident light. On some 
occasions we have the light from a blue sky; on other oc- 
casions the sky is invested with clouds of various depths of 
grey, or sometimes tinged by the sun with a variety of tints, 
from yellow to red ; while the light of the sun itself is fre- 
quently yellow or orange. All these variations of light are 
likely to have some influence in our judgment of colour, 
especially when the tints to be compared are light. Of the 
disturbing influence of colour in the incident light, anyone 
may convince himself by comparing yellows on a morning 
when the sky is enveloped in a yellow fog. In some experi- 
ments which I made with bichromate of potash during such 
fogs, I found it much more difficult to decide at what depth 
equality of colour was effected ; the disc in the stronger so- 
lution could be moved through a very considerable range 
without any change of colour being perceived. A similar 
