222 NATURAL HTST0RT SOCIETY OF DUBLIN. 



of Trinity College. In these calculations he had no data taken from 

 the eye of a Seal, and was obliged to take those supplied by the human 

 eye. The depth of the human eye from the front of the cornea to the 

 retina is 0-95 of an inch. Assuming that the accommodation of the 

 human eye necessary for the purpose of vision in water is accomplished 

 by the action of the recti muscles, the eye would have to be pulled out 

 by those muscles from a diameter of less than an inch to 2*75 inches. 

 That showed it to be physically impossible that the necessary change 

 could be produced by the simple elongation of the chamber. The first 

 mathematician who ever directed his attention to this curious and im- 

 portant question was Kepler. Modern researches have not added much to 

 his investigation of the subject ; and he asserted that the accommodation 

 of the human eye must be due to one of the following causes : — I . Change 

 in the cornea; 2. Lengthening or shortening of the eye; 3. Change in 

 the position of the crystalline lens ; 4. Change in the shape of the crys- 

 talline lens. Kepler himself decided, so far as the human eye was con- 

 cerned, in favour of the solution which derives the power of change 

 from the position of the crystalline lens. Mr. Wilson had directed at- 

 tention to the fact that there was a zone of minimum strength in the eye 

 of the Seal. "Whether the effect of that zone is, as Mr. Wilson states, 

 to elongate the eye when the recti muscles press upon it, or to shorten 

 it, as Dr. Bennett thinks, is a matter of indifference ; for it resolves itself 

 into a question of whether the Seal's eye is accommodated from water 

 to air by the shortening of the globe, or from air to water by the 

 lengthening of it. The aqueous and the vitreous humours are so 

 nearly coincident in refractive power with the water in which the 

 Seal lives, that practically his eye when he dives may be considered 

 as reduced to its crystalline lens only. Applying that assumption to 

 the human eye, Dr. Lloyd has calculated that a man, in order to see 

 clearly in water, would require to wear double convex spectacles of 

 crown glass, having a radius of one-third of an inch both ways. 

 The amount of alteration, therefore, necessary for the change from 

 air to water, was so enormous as to render it indispensable that what- 

 ever solution of the question was to be accepted should be decisively 

 made out. Kepler's first solution explains nothing, for no change in 

 the shape of the cornea under water would alter the phenomena. The 

 fact that an elongation of the eye from one inch to two inches and 

 three-quarters was physically impossible excluded his second solution ; 

 therefore the third and fourth, as to the position or the shape of the 

 crystalline lens, only remained. As long as we confine ourselves to 

 the human eye, Kepler's solution, by the shifting backwards and forwards 

 of the crystalline lens, is not to be disputed. He shows that a very small 

 change in the position of that lens along a line directed to and from the 

 object would be sufficient. But this explanation will not answer for 

 animals changing from water to air. Nothing is left, then, for their 

 case, but the fourth supposition, that of the change in the shape of the 

 crystalline lens. Even this must take place on a far greater and more 



