We have yet to speak of the very remarkable inversion of this appearance, as related by men of science. If, namely, instead of looking at the colours by a reflected light, we examine them by a transmitted light, the opposite colours are said to appear, and in a mode corresponding with that which we have before described as physiological; the colours evoking each other. Instead of blue, we should thus see red-yellow; instead of red, green, &c., and vice versâ. We reserve experiments in detail, the rather as we have ourselves still some doubts on this point.
If we were now called upon to give some general explanation of these epoptical colours, as they appear under the first condition, and to show their connexion with the previously detailed physical phenomena, we might proceed to do so as follows:—
The glasses employed for the experiments are to be regarded as the utmost possible practical approach to transparence. By the intimate contact, however, occasioned by the pressure applied to them, their surfaces, we are persuaded, immediately become in a very slight degree dimmed. Within this semi-transparence the colours immediately appear, and every circle comprehends the whole scale; for when the two opposites, yellow and blue, are united by their red extremities, pure red appears: the green, on the other hand, as in prismatic experiments, when yellow and blue touch.
We have already repeatedly found that where colour exists at all, the whole scale is soon called into existence; a similar principle may be said to lurk in the nature of every physical phenomenon; it already follows, from the idea of polar opposition, from which an elementary unity or completeness results.
The fact that a colour exhibited by transmitted light is different from that displayed by reflected light, reminds us of those dioptrical colours of the first class which we found were produced precisely in the same way through semi-opacity. That here, too, a diminution of transparency exists there can scarcely be a doubt; for the adhesion of the perfectly smooth plates of glass (an adhesion so strong that they remain hanging to each other) produces a degree of union which deprives each of the two surfaces, in some degree, of its smoothness and transparence. The fullest proof may, however, be found in the fact that in the centre, where the lens is most strongly pressed on the other glass, and where a perfect union is accomplished, a complete transparence takes place, in which we no longer perceive any colour. All this may be hereafter confirmed in a recapitulation of the whole.