There is an idea that the phenomena of sound bear a relation to those of light,--that there exists a resemblance between the chromatic and the diatonic scales. Sound, we know, is conveyed by the beating of material particles--the air--upon the auditory membrane of the ear, which have been set in motion by some distant disturbance of the medium through which it passes. Light has been supposed to act on the optic nerve in the same manner. If we imagine colour to be the result of vibrations of different velocities and lengths, we can understand that under some of these tremors, first established on the nerves, and through them conveyed to the brain, sensations of pain or pleasure may result, in the same way as sharp or subdued sounds are disagreeable or otherwise. Intensely coloured bodies do make an impression upon perfectly blind men; and those who, being born blind, know no condition of light or colour, will point out a difference between strongly illuminated red and yellow media. When the eyes are closed we are sensible to luminous influence, and even to differences of colour. We must consequently infer that light produces some peculiar action upon the system of nerves in general; this may or may not be independent of the chemical agency of the solar radiations; but certainly the excitement is not owing to any calorific influence. The system of nerves in the eye is more delicately organized, and of course peculiarly adapted to all the necessities of vision. Thus far some analogy does appear to exist between light and sound; but the phenomena of the one are so much more refined than those of the other--the impressions being all of them of a far more complicated character, that we must not be led too far by the analogical evidence in referring light, like sound, to mere material motion. It was a beautiful idea that real impressions of external objects are made upon the seat of vision, and that they are viewed, as in a picture, by something behind the screen,--that these pictures become dormant, but are capable of being revived by the operations of the mind in peculiar conditions; but we can only regard it as a philosophical speculation of a poetic character, the truth or falsehood of which we are never likely to be enabled to establish.[108] That which sees will never itself be visible. The secret principle of sensation,--the mystery of the life that is in us,--will never be unfolded to finite minds.