77. Before speaking of the relation of my experiments to those made by others on the contrast of colours, we must distinguish three classes of contrast. The _first_ includes those relating to the contrast which I term _simultaneous_; the _second_ to that which I term _successive_; and the _third_ to that which I term _mixed_. 78. _Simultaneous contrast of colours_ includes that phenomena of modification which objects variously coloured seem to undergo in physical composition, and in the depth of tone of their respective colours, when seen at the same time. 79. _Successive contrast of colours_ includes the phenomena observed when the eyes having looked for some time at one or more coloured objects, perceive, after having ceased to look at them, images of those objects, presenting the colour complementary to that of the actual object. 80. This distinction also facilitates the understanding of the phenomena which maybe called _mixed contrast_; for, the retina having seen a certain colour for some time, has an aptitude to see for a further time the complementary of that colour, as well as any new colour presented by an external object; the sensation perceived being the result of this new colour, and the complementary of the first. For want of this distinction, one of the branches of optics most fertile in applications, has not been treated with the clearness and precision which are necessary to show its importance to those who have not experimented upon it. 81. The following is a simple mode of observing _mixed contrast_:—Having closed one eye, the right for example, look steadily with the left at a sheet of paper of a colour A; when this colour seems to become dull, look immediately at a sheet of paper of the colour B, the eye then has the impression produced by the mixture of this colour B with the complementary (C) of the colour A. 82. To be convinced of this mixed sensation, it is only necessary to shut the left eye, and to look at the colour B with the right eye; when the sensation perceived is not only that of the colour B, but it may appear modified in the contrary way to that of the mixed sensation C + B, or what is the same, it appears to be rather A + B. 83. Upon shutting the right eye, and looking again at the colour B with the left eye, and that many times in succession, different sensations are successively perceived, but more and more feebly, until at length the left eye returns to its normal state. 84. I advise any person who thinks that one of his eyes is more able to perceive colours than the other, to look at a sheet of paper alternately with the right and left eye; if the sensations of each are identical, he may conclude that he has deceived himself. And even if the sensations be different, the experiment should be repeated several times, successively, for the difference observed in a single experiment might be occasioned by one of the eyes having been previously modified or fatigued.