But about colour? Distinct colour-waves have (as we said before) distinct velocities, and are therefore objectively distinguished even in the inorganic universe. They also act differently upon the growth of animals and plants,--and other distinctions might be added. The _sensation_ is, however, our point,--the special thing called colour both by careful speakers and in child parlance,--what do we really know about this? Little indeed except as an impression received by sight. The man born in complete blindness taking a piece of red cloth to examine, described the fabric minutely; but, when asked if he could say anything about its redness, likened that "hue angry and brave" to the sound of a trumpet. A simile most conclusive,--suggested probably by his having often heard of certain "scarlet-coated gentry";--and proving beyond doubt that colour is non-existent in the sensory of a person affected from birth by a deep-seated lesion. To one less thoroughly blind, spectra are possible, and red light may be produced under pressure. It thus appears, that colour must be perceived by a nervous _substratum_, called the rod and cone layer; and hence we explain our power of distinctly seeing the blood-vessels of the retina lying immediately before that structure.[97] These curiosities, of vision shew that our powers of verifying shape are superior to our powers of verifying colour;[98] add, too, that the latter sensation, (as an idealist might maintain,) is known to be sometimes unreal, since it occurs without a coloured object. We can produce it, for instance, by gazing at the sun--a phenomenon mentioned by Aristotle. But then, this ideal sense-affection ranges with a variety of others, which taken together constitute a very much wider law. Not to mention many superinduced mental states, we see light under the influence of a touch or blow,--of electricity,--of chemicals, such as narcotic medicines, which attack the nervous system. We hear sound under like appliances stimulating the auditory nerve. And the whole of these affections are to be explained by another Aristotelian doctrine, extended and pushed to its consequences. Special senses have their own proper faculties, and when called into action each exerts its power within its special province. Had Aristotle dissected out nerve-fibres, he might have discovered the larger empire of specialty now known to our anatomists.[99]