45'1 PHILOSOPHICAL TRANSACTIONS. [aNNO 17Q5. 



prosecute so arduous an inquiry. Tine progress Mr. Hunter had made in this in- 

 vestigation enabled him to prove the crystalline humour of the eye to be laminated 

 and the laminae to be composed of fibres ; but the use to which these fibres are 

 applied in the economy of the eye he had not ascertained, though several experi- 

 ments were instituted with that view : his opinion was certainly in favour of their 

 being muscular, for the purpose of adjusting the eye to different distances by their 

 contraction and relaxation. 



Being unwilling that a subject on which Mr. Hunter had so publicly given his 

 opinion should remain in an unfinished state, I requested the President's per- 

 mission to be allowed to give the Croonian lecture for the present year, as it 

 would afford me an opportunity of weighing with impartiality the facts already 

 ascertained, and of endeavouring by my own labours to add to their number. In 

 prosecuting this inquiry, I consider myself to have been particularly fortunate in 

 having had the assistance of Mr. Ramsden. It was a subject connected with his 

 own pursuits, and one which had always engaged his attention ; he was therefore 

 peculiarly fitted, both by his own ingenuity and knowledge in optics, for such 

 an investigation. In conversing on the different uses of the crystalline humour, 

 he made the following observations. 



He said, that as the crystalline humour consists of a substance of different 

 densities, the central parts being the most compact, and from thence diminishing 

 in density gradually in every direction, approaching the vitreous humour on one 

 side, and the aqueous humour on the other, its refractive power becomes nearly 

 the same with that of the 2 contiguous substances. That some philosophers 

 have stated the use of the crystalline humour to be, for accommodating the eye 

 to see objects at different distances ; but the firmness of the central part, and the 

 very small difference between its refractive power near the circumference and that 

 of the vitreous, or the aqueous humour, seemed to render it unfit for that pur- 

 pose; its principal use rather appearing to be for correcting the aberration arising 

 from the spherical figure of the cornea, where the principal part of the refraction 

 takes place, producing the same effect that in an achromatic object glass we ob- 

 tain in a less perfect manner, by proportioning the radii of curvature of the differ- 

 ent lenses. In the eye, the correction seems perfect, which in the object glass 

 can only be an approximation, the contrary aberrations of the lenses not having 

 the same ratio ; so that if this aberration be perfectly corrected at any given dis- 

 tance from the centre, in every other it must be in some degree imperfect. 



Pursuing the same comparison : in the achromatic object glass, we may con- 

 ceive how much an object nnist appear fainter from the great quantity of light 

 lost by reflection at the surfaces of the different lenses, there being as many pri- 

 mary reflections as there are surfaces ; and it would be fortunate if this reflected 

 light was totally lost. Part of it is again reflected towards the eye by the interior 



