14. This effect of rendering the organs of perception insensible to weaker excitations, by strongly exciting those organs, is analogous to the constitution of the human frame in many instances. Accustom the eye either to much light, or to intense colours, and, for a time, it will hardly discern anything 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 produce any effect, owing to the previous excitation of a stronger red *. *[This distinction should always be kept in mind, for, unless the eye has been absolutely injured or weakened 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 it 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 are 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 on the other senses.]
