107. Well, in order to get at the meaning we may, indeed, now appeal to physical science, and ask her to help us. How many manner of eyes are there? You physical-science students should be able to tell us painters that. We only know, in a vague way, the external aspect and expression of eyes. We see, as we try to draw the endlessly-grotesque creatures about us, what infinite variety of instruments they have; but you know, far better than we do, how those instruments are constructed and directed. You know how some play in their sockets with independent revolution,--project into near-sightedness on pyramids of bone,--are brandished at the points of horns,--studded over backs and shoulders,--thrust at the ends of antennæ to pioneer for the head, or pinched up into tubercles at the corners of the lips. But how do the creatures see out of all these eyes? 108. No business of ours, you may think? Pardon me. This is no Siren's question--this is altogether business of ours, lest, perchance, any of us should see partly in the same manner. Comparative sight is a far more important question than comparative anatomy. It is no matter, though we sometimes walk--and it may often be desirable to climb--like apes; but suppose we only _see_ like apes, or like lower creatures? I can tell you, the science of optics is an essential one to us; for exactly according to these infinitely grotesque directions and multiplications of instrument you have correspondent, not only intellectual but moral, faculty in the soul of the creatures. Literally, if the eye be pure, the body is pure; but, if the light of the body be but darkness, how great is that darkness! 109. Have you ever looked attentively at the study I gave you of the head of the rattle-snake? The serpent will keep its eyes fixed on you for an hour together, a vertical slit in each admitting such image of you as is possible to the rattlesnake retina, and to the rattlesnake mind. How much of you do you think it sees? I ask that, first, as a pure physical question. I do not know; it is not my business to know. You, from your schools of physical science, should bring me answer. How much of a man can a snake see? What sort of image of him is received through that deadly vertical cleft in the iris;--through the glazed blue of the ghastly lens? Make me a picture of the appearance of a man, as far as you can judge it can take place on the snake's retina. Then ask yourselves, farther, how much of speculation is possible to the snake, touching this human aspect?