720 DR. 6. L. JOHNSON ON THE SBAL'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 we find 

 in most text-books, the membrana nictitans is never used for 

 brushing away foreign bodies as in birds \ 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 me 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 covei'ings. The posterior half 

 is lined with the light-sensitive retina, whUe 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, we 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 we 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). We shall thus, under water, be quite unable to see 

 anything around us distinctly, and that for a very simple reason. 

 "We 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 ^0-61 mm. = l-991: inches. 

 Since the 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 He 50-61 mm. — ] 5-6 mm., or 35 mm., behind the retina ; in 

 other words, we need the addition of a convex lens of something 



^ Exceptions to this rule occur in the Ungulata and certain other grass- 

 feeding animals. 



