"And what progress have we made in our task of explaining Sight? It might seem that we are farther off than ever; the riddle only more complicated, and less hope than ever of finding out the answer. The reader may perhaps feel inclined to reproach Science with only knowing how to break up with fruitless criticism the fair world presented to us by our senses, in order to annihilate the fragments." (p. 269.)
How triumphant does Idealism now appear! How little trustworthy that boasted sense of which mankind have constantly said, "seeing is believing," although an apostle and philosophers innumerable have put the two in opposition!
Perhaps, however, instead of leading to a "triumph of Idealism," the paradoxes and incongruities—in a word, the vast accumulation of the Unknowable—belonging to eyesight considered as a Sensation, must be allowed to land us on the shore of a far-stretching Scepticism illimitable to the mind's eye. And this seems to be the eminent writer's own final opinion.[123] So, too, it will always appear when the case is fairly argued out; and that for the reasons adduced in our text. The course of argument there pursued was adopted before the Professor's book came to hand; but we have now added some extracts from his pages in the shape of footnotes, and have given references to other interesting topics touched upon by him.
For our purpose, however, it is necessary in some degree to disregard the variety of those topics, and fix our attention upon the conclusive issue. It is plain, that respecting our senses, as well as our other primary sources of information, the limits of what we can completely explain are very narrow. Yet each for himself and all of us for our race must needs every day accept and act upon this limited and imperfect kind of knowledge about what most essentially concerns our actions as well as our speculations.
Several strong examples of such incompleteness are given by Helmholtz in his scientific inquiry into the rationale of the visual sense-impressions. We observe, for instance, in his chapter on Sensation (p. 236 seq.) that all light-waves are the same in kind of movement, but differ in size as widely as the ripples on a sea-beach (round which happy children play) differ from the vast Atlantic ship-engulfing billows sixty or a hundred feet apart. All these undulations are similar in respect of reflection, refraction, interference, diffraction, and polarisation, as well as in their production of heat.[124] Now, it is the interpretation of such movements into its own language by which our eye gives us the sensation of colour. Yet this power of interpretation is curiously limited—it does not appreciate the gentler ripples of the light-waves—it does not reach to their mightier undulations. Consequently, there may be tender colour-delicacies adorning the Universe, completely incognisable by us, and there may be also glows and intensities of light-beams magnificently resplendent, and unspeakably grand in tone, of which we can through our visual apparatus form no possible conception. Thus, our eye translates some waves into a language which we call colour, but its scholarship is limited. A certain number of signs it catches and interprets, the rest lie altogether outside its ken. The Sun's softer light-harmonies, and his most awful emanations of beauty remain equally unknown.
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.)