181 LECTURE VI. THE EYE. THE discovery of glass led, as we have seen, to consequences far exceeding, in importance and beneficial influence, any thing that could have been imagined by those who first combined the ashes of vegetables with pounded rock or sand. In tracing the progress of the art of glassmaking, we have observed its application to the structure of those curious instruments, which have unveiled new worlds to the eye of the astronomer, and enabled him to compare the phenomena he observes in them, with appearances in our own planet, of which the causes are well known. How great must have been the pleasure of Herschel, when, directing his 182 THE EYE, AN OPTICAL INSTRUMENT, telescope to Mars, he saw the polar regions of that planet, overspread with a whiteness resembling the mantle of snow which covers the corresponding part of our globe; or, when he watched, for successive days, those spots of glowing brightness, which seemed to be the burning craters of volcanoes, situated among the lofty mountains of the moon!* All these, and yet greater discoveries, and all the pleasure derived from viewing the minute wonders of creation, we owe to instruments constructed by the ingenuity of man. But there is another optical instrument, nearly allied to the telescope, still more nearly to the camera-obscura, of which, as yet, we have taken no notice, though without it all the others would be useless; and as we began this little series of Lectures, by regarding every thing which claims our notice, as included in one or other of two great classes, as being either a natural or an artificial production, suppose we conclude them by the examination of an optical instrument, curious, beautiful, per * Ency. Brit. COMPARED WITH THE CAMERA. 183 fect, far excelling all that has been invented by the art of man—an instrument which became such as we see it by the contrivance of the Divine Artificer, whose power and goodness are as clearly exhibited in the minute perfection, as in the grandeur or immensity of his works. I suppose it is scarcely needful for me to tell you, that the optical instrument we are about to examine is the eye; but it is very likely that you are surprised at hearing me call it an instrument. We shall see, as we proceed, whether the term is not strictly applicable. In the first place, we will compare the eye with the camera-obscura; beginning with the human eye, as most interesting to us, and afterwards noticing some peculiarities in the eyes of animals. The striking resemblance of the eye to the camera-obscura, consists in the light being admitted through a double convex lens to a dark chamber, in which the image is formed by re flection. 184 STRUCTURE OF THE EYE. The eye is such a chamber, of a globular form, and about the size of a walnut. In describing the different coats which compose the walls of this chamber, I shall be obliged to make use of the Greek and Latin words which have been adopted as the names of those coats, but by carefully explaining their meaning, as we proceed, I think you will not find any difficulty in understanding them. We have here two drawings of the eye, representing, in front and in profile, that part which is visible when the eyelids are open. The figure beneath is a section of the eye, showing the internal structure, as it would appear if the eye were cut down the middle, and half of it placed before us in the same position as the profile of the eye above. Do not perplex yourselves by attempting to understand the section at once; each line will be explained as we have occasion for it. If you press your finger round the socket of your eye, beneath the arch of the brow, and above the cheek-bone, you will be sensible that |