The human eye has been rightly called the “masterpiece of divine mechanism;” its structure is complicated, yet all the adjustments of its parts are as simple as they are perfect. The eye-ball consists of four coats. The cornea is the transparent coat in front of the globe; it is the first optical surface, and this is attached to the sclerotic membrane, filling up the circular aperture in the white of the eye; the choroid coat is a very delicate membrane, lining the sclerotic, and covered with a perfectly black pigment on the inside; and close to this lies the most delicately reticulated membrane, the retina, which is, indeed, an extension of the optic nerve. These coats enclose three humours,--the aqueous, the vitreous, and the crystalline humours. 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:--