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Show 720 DR. G. L. JOHNSON ON THE SEAL'S EYE. [Nov. 21, particularly large for the size of the animal, being slightly larger than our own. The sclerotic is chalky white, almost entirely free from blood-vessels, and the cornea very large, round, and of great convexity. This latter point is very remarkable for a reason which I will point to later on. The iris is of a rich yellow-brown colour and contracts rapidly to light. The eye is capable of being partly retracted into the orbit and is protected by a well-developed membrana nictitans. I have repeatedly amused myself by making experiments to ascertain how far the m. nictitans can be drawn over the eye of the Seal, but I could never get it to extend over more than a third of the distance across the cornea of its own accord, although I have tried, by pinching the conjunctiva or by placing hairs and other foreign bodies on the cornea, to stimulate it to further action ; but this always failed to have any effect, as, contrary to what w e find in most text-books, the membrana nictitans is never used for brushing away foreign bodies as in birds1. I hope in a future paper to discuss the action and purpose of the membrana nictitans in various families of the Mammalia, but space compels m e to confine myself in this paper to the apparatus more immediately concerned in vision. Before going further I will, with your permission, pass a few remarks on the nature of vision in ourselves. The human eye, as we are all aware, is a nearly spherical expansion of the optic nerve and its connective-tissue coverings. The posterior half is lined with the light-sensitive retina, whde the anterior portion, together with the contents of the globe, form a dioptric apparatus for the convergence of rays on its surface. This dioptric system is made up of four distinct media : the cornea, the aqueous humour, the lens, and the vitreous body. This optical system consists then of the transparent media and the refracting surfaces which separate them from the air and from each other. Now if we examine their refractive indices, w e shall find that, excepting the lens, all the media have the same refractive index, viz. 1-3365. But this is the refractive index of sea-water, so that if w e plunge our heads under the waves our dioptric apparatus becomes at once simplified down to a single lens in front of a sensitive surface (the retina). W e shall thus, under water, be quite unable to see anything around us distinctly, and that for a very simple reason. W e know that in a state of rest parallel rays come to a focus on the retina of a normal eye. Now in a state of rest the crystalline lens has in aqueous humour (or, what amounts to the same thing, in sea-water) a mean focal distance of 50*61 m m . = 1-994 inches. Since tbe distance between the optical centre of the lens and the retina amounts to 15-62 mm., it is clear the image for parallel rays must lie 50-61 m m . -15-6 mm., or 35 mm., behind the retina; in other words, w e need the addition of a convex lens of something 1 Exceptions to this rule occur in tbe Ungulata and certain other grass-feeding animals. |
