on Muscular Motion. 
9 
tion, by which the degree of adjustment produced by each of 
these changes may be ascertained. This he has promised to 
render more correct ; and also to institute a series of experi- 
ments by which the effects of the motion of the lens may be 
more accurately determined. From Mr. Ramsden's compu- 
tation, the increase of curvature of the cornea appears capable 
of producing one-third of the effect ; and the change of place 
of the lens, and elongation of the axis of vision, sufficiently 
account for the other two-thirds of the quantity of adjustment 
necessary to make up the whole. 
Having explained the mode by which the axis of vision can 
be elongated, and the convexity of the cornea increased, in the 
human eye, for the purpose of its adjustment, I was desirous of 
applying these observations to the eyes of other animals, that 
I might see whether their different structures would admit of 
the necessary changes, for producing an adjustment to different 
distances in the same way. 
As many animals are known tohave their vision distinct at very 
different distances, it .appeared that much information might 
be gained by examining the structure of the eyes of those 
whose range of vision varies most from that of the human eye. 
Quadrupeds in general must have their eyes fitted to see 
very near objects, as many of them collect their food with 
their mouths, in which action the objects are brought very 
close to the eye. Birds are under the same circumstances in 
a still greater degree with respect to their food ; but from 
their mode of life, they also require the power of seeing objects 
at a great distance. Fishes, from the nature of the medium in 
which they live, must have some other mode of adj usting the eye, 
MDCCXCVI. C 
