And another limitation has been imposed upon our optical apparatus. For a perception of heating powers belonging to colour-waves the eye refers us to the skin;--and as to their chemical powers we are only just now discovering the instruments fitted for their true appreciation. Skilful, too, and yet at the same time very skill-less, is the divination into sunlight given us by our human eyes;--sunlight, that is to say, as a general resultant in its whiteness. For, if our eyes, keen and susceptible to us perfect clearness, attempt to analyze white light into its factors and elements, their resolving faculty manifests still more blank inabilities. And they fail also in examining certain colours:-- "The most striking difference," writes Helmholtz, "between the mixture of pigments and that of coloured light is, that while painters make green by mixing blue and yellow pigments, the union of blue and yellow rays of light, produces white.... In general, then, light, which consists of undulations of different wave-lengths, produces different impressions upon our eye, namely, those of different colours. But the number of hues which we can recognise is much smaller than that of the various possible combinations of rays with different wave-lengths which external objects can convey to our eyes. The retina cannot distinguish between the white which is produced by the union of scarlet and bluish-green light, and that which is composed of yellowish-green and violet, or of yellow and ultramarine blue, or of red, green, and violet, or of all the colours of the spectrum united. All these combinations appear identically as white; and yet, from a physical point of view, they are very different. In fact, the only resemblance between the several combinations just mentioned is, that they are indistinguishable to the human eye. For instance, a surface illuminated with red and bluish-green light would come out black in a photograph; while another lighted with yellowish-green and violet would appear very bright, although both surfaces alike seem to the eye to be simply white. Again, if we successively illuminate coloured objects with white beams of light of various composition, they will appear differently coloured. And whenever we decompose two such beams by a prism, or look at them through a coloured glass, the difference between them at once becomes evident. "Other colours, also, especially when they are not strongly pronounced, may, like pure white light, be composed of very different mixtures, and yet appear indistinguishable to the eye, while in every other property, physical or chemical, they are entirely distinct." (pp. 239-241.)