ON THE MECHANISM OF THE EYE. 



603 



ascertaining, upon inspection, the exact focal 

 distance of every eye, and the remedy for 

 its imperfections. Thirdly, to show the ac- 

 curate adjustment of every part of the eye, 

 for seeing with Tjistinctness the greatest pos- 

 sible extent of objects at the same instant. 

 Fourthly, to measure the collective disf)ersion 

 of coloured rays in the eye. Fifthly, by 

 immerging the eye in water, to demonstrate 

 that its accommodation does not depend on 

 any change in the curvature of the cornea. 

 Sixthly, by confining the eye at the extre- 

 mities of its axis, to prove that no material 

 alteration of its length can take place. Se- 

 venthly, to examine what inference can be 

 drawn from the experiments hitherto made 



on persons deprived of the lens ; to pursue the 

 inquiry, on the principles suggested by Dr. 

 Porterfield ; and to confirm his opinion of 

 the utter inability of such persons to change 

 the refractive state of the organ. Eighthly, 

 to deduce, from the aberration of the lateral 

 ra_ys, a decisive argument in favour of a 

 change in the figure of the crystalline; to 

 ascertain, from the quantity of this aberration, 

 the form into wliich the lens appears to be 

 thrown in my oWn eye, and the mode by 

 which the change must be produced in that 

 of every other person. And I flatter myself, 

 that I shall not be deemed too precipitate, 

 in denominating this series of experiments- 

 satisfactorily demonstrative. 



