The eye, in its more superficial mechanical arrangements, presents exactly the same character as a camera obscura, the cornea and crystalline lens receiving the images of objects refracting and inverting them; but how infinitely more beautiful are all the arrangements of the organ of vision than the dark chamber of Baptista Porta![106] The humours of the eye are for the purpose of correcting the aberrations of light, which are so evident in ordinary lenses, and for giving to the whole an achromatic character. Both spherical and chromatic aberration are corrected, the latter not entirely, and by the agency of the cornea and the crystalline lens perfect images are depicted on the retina, in a similar way to those very charming pictures which present themselves in the table of the camera obscura.
The seat of vision has been generally supposed to be the retina; but Mariotte has shown that the base of the optic nerve, which is immediately connected with the retina, is incapable of conveying an impression to the brain. The choroid coat, which lies immediately behind the retina, is regarded by Mariotte and Bernoulli as the more probable seat of vision. The retina, being transparent, offers no obstruction to the passage of the light onward to the black surface of the choroid coat, from which the vibrations are, in all probability, communicated to the retina and conveyed to the brain. Howbeit, upon one or the other of these delicate coats a distinct image is impressed by light, and the communication made with the brain possibly by a vibratory action. We may trace up the phenomena of vision to this point; we may conceive undulations of light, differing in velocity and length of wave, occasioning corresponding tremors in the neuralgic system of the eye; but how these vibrations are to communicate correct impressions of length, breadth, and thickness, no one has yet undertaken to explain.
It has, however, been justly said by Herschel:—
“It is the boast of science to have been able to trace so far the refined contrivances of this most admirable organ, not its shame to find something still concealed from scrutiny; for, however anatomists may differ on points of structure, or physiologists dispute on modes of action, there is that in what we do understand of the formation of the eye, so similar, and yet so infinitely superior to a product of human ingenuity; such thought, such care, such refinement, such advantage taken of the properties of natural agents used as mere instruments for accomplishing a given end, as force upon us a conviction of deliberate choice and premeditated design, more strongly, perhaps, than any single contrivance to be found whether in art or nature, and renders its study an object of the greatest interest.”[107]
Has the reader ever asked himself why it is, having two eyes, and consequently two pictures produced upon the tablets of vision, that we see only one object? According to the law of visible direction, all the rays passing through the crystalline lenses converge to one point upon the retina,—and as the two images are coincident and nearly identical, they can only produce the sensation of one upon the brain.
When we look at any round object, as the ornamented moderator lamp before us, first with one eye, and then with the other, we discover that, with the right eye, we see most of the right-hand side of the lamp, and with the left eye more of the left-hand side. These two images are combined, and we see an object which we know to be round.
This is illustrated in a most interesting manner by the little optical instrument, the Stereoscope. It consists either of two mirrors placed each at an angle of 45°, or of two semi-lenses turned with their curved sides towards each other. To view its phenomena, two pictures are obtained by the camera obscura on photographic paper of any object in two positions, corresponding with the conditions of viewing it with the two eyes. By the mirrors or the lenses these dissimilar pictures are combined within the eye, and the vision of an actually solid object is produced from the pictures represented on a plane surface. Hence the name of the instrument; which signifies, Solid I see.
Analogy is often of great value in indicating the direction in which to seek for a truth; but analogical evidence, unless where the resemblance is very striking, should be received with caution. Mankind are so ready to leap to conclusions without the labour necessary for a faithful elucidation of the truth, that too often a few points of resemblance are seized upon, and an inference is drawn which is calculated to mislead.
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.