



Monday, \6th December 1844. 

 Sir GEO. MACKENZIE, Bart., in the Chair. 



The following Communications were read : — 



1 . On a Possible Explanation of the Adaptation of the Eye to 

 Distinct Vision at Different Distances. By Profes- 

 sor Forbes. 



The idea suggested in this paper, occurred to the author three 

 years ago, from reflecting that the destruction of spherical aber- 

 ration in the eye might be effected by a modification of the curva- 

 ture of the lens, as well as by the variable density which it is 

 known to possess, and which has usually been accounted for as in- 

 tended for that purpose. 



The author considering the probability to be almost infinite 

 against the sphericity of the surfaces (a necessary evil in our instru- 

 ments, but inexplicable in a natural organ), a conviction which he 

 afterwards found to be reduced to certainty by experiments which 

 have actually been made on the figure of the lens — he conceived 

 that the variable density of this part of the eye must have some other 

 cause. He considered it likely that it might conti'ibute to the focal 

 adjustment of the eye in the following way : — The lens is composed 

 of coats more firm and tenacious, as well as more refractive, towards 

 the centre, and less so at the sides. These coats are also nearly 

 spherical in the centre, forming a nucleus of considerable resistance. 

 Hence the author supposes, that if the lens be compressed in any 

 manner by a uniform hydrostatic pressure, it will yield most readily 

 in a plane at right angles to the axis of vision, and hence the lens 

 will become more spheroidal, and, consequently, more refractive ; 

 that is, adapted for the vision of objects at small distances. The 

 hydrostatic pressure in question is believed to be conveyed from the 

 humours of the eye, between which the lens is delicately suspended, 

 and to originate in the compressing action of the muscles which 

 move the eye-ball acting simultaneously on the tough sclerotic coat. 



The author thus sums up the evidence which he thinks gives pro- 

 bability to this explanation : — 



1. The form of the surfaces of the lens might have been such as 

 to correct aberration without any variation of density whatever. 

 But, on the contrary, it has a form which exaggerates the ordinary 

 ."spherical aberration. A form which, therefore, appears tu bo 



