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human    frame in many instances.       Accustom    the eye either to much light, or to
intense colours, and for a time it will hardly discern any thing by a dull light, or by
feeble colours, provided the feeble colours be of the same kind with the previous
strong ones. Thus, after it has been excited by an intense red, for example, it will
for a time be insensible to weak red colours, yet it will still easily perceive a weak
green, or blue, &c. as in the instance before us respecting the shadow D. V. where
the green part of the compound still affects the eye, after the red-has ceased to pro-
duce any effect, owing to the previous excitation of a stronger red.b
15. Nor is this the case only with the eye, it is the same with every other sense;
precise instances of this kind in regard to the taste, the smell, the touch, &c. will
occur plentifully to every one.
16. I consider this solution of the appearances of the colours as perfectly satisfactory,
Here it is applied only to one instance, but it is equally applicable to all the rest; and
it appears to me to account for all the difficulties which seem to have embarrassed
Count Rumfo~d, in his very ingenious and entertaining paper, Phil. Trans. 1794,
p. 107.   Also in Dr. Priestley's History of Optics, p. 436, there is a curious Chapter,
containing the observations of philosophers on blue and green shadows; the true
cause of these shadows is not, I think, there mentioned; and it may be entertaining
to read that Chapter with these principles in the mind.
17. When the sun has been near setting in a summer evening, I have often
9bserved most beautiful blue shadows upon a white marble chimney-piece. In this
case, the weak white light of the evening which illumines the shaded part of the
mnarble, is to be considered as compounded of two colours, orange and blue.              The
direct orange rays of the sun at this time, render the orange part invisible, and leave
the blue in perfection.
i8. And in the same way is to be explained that beautiful and easy experiment
mentioned by Count Rumford, p. 103. PhiL Trans.. 1794, where a burning candle in,
b This distinction should always be kept in mind, for unless the eye has been absolutely injured or weakene4
by excessive excitation, there is reason to believe that strong excitations of it, whether immediately preceding
Weaker ones, or contemporaneous with them, much improve its sensibility in regard to those weaker ones, provided
only that they be of a different class. If the eye has been excited by a lively red colour, it will scarcely perceive
a weak red, but i't will perceive a weak green much better on account of the previous excitation by the strong
red; and the reason may be, that in looking at a red colour, the eye wastes none of that nervous sensibility which:
is necessary for its seeing a green colour; and the same reasoning holds in all other cases where the colours am
contrasts to each other. For such colours seem incapable of mixing with each other, in the proper sense of the
word, as when red and yellow are mixed together, and produce a compound evidently partaking of the obvious
properties of the two ingredients. When contrasts are mixed together, as red and green, these colours seem
destructive of each other, and effect a compound approaching to whiteness. Similar observations may be made
o, the other senses.
2 F