The Laws Which Regulate The Colours Of The Ocular Spectra, above alluded to, are curious, and not so easily explained as we are often asked to believe. If a brightly-coloured object on a white ground be steadily gazed at, on looking away from it to the surface on which it lies, or, still better, looking to a dark surface, or shutting the eyes, the image of the object remains before the sight, in the complementary colour—that is to say, in the colour which, added to that of the object gazed on, would make white light. If the object be red. the spectrum will be green, and if the object be blue, there will be an orange spectrum; and the explanation commonly given is, that the part of the retina on which the coloured rays have fallen becomes by exhaustion less affected by the rays of the same colour in the light around, while it is affected by all the colours entering into white light. The following circumstances, however, show that explanation to be insufficient:— 1. The brightest complementary-coloured spectrum is obtained by shutting the eyes, or looking into total darkness. 2. While continuing to gaze on the coloured object, a ring of light more brilliant than the surrounding surface makes its appearance round about, and the spectrum seen on the white surface is of the same shade as that ring of light. 3. If the object gazed at be on a dark ground, the ring about it will be, not one of light, but of greater darkness; and the spectrum, if cast on that ground, will be of the same shade. 4. If a very brilliant spectrum be obtained, its colour can be temporarily changed by pressure on the shut eyes, and will again return.