VOL. LXXXIV.] PHILOSOPHICAL TRANSACTIONS. 413 



3dly, May it not in part be owing to the loss of this combined action of the 

 external muscles, and the difficulty of recovering it, that the operation of couch- 

 ing is sometimes unsuccessful, especially when the cataract has been of long 

 standing ? This cannot be attributed to the iris, for it perhaps dilates and con- 

 tracts as before : nor to the muscles of the lens, for they are removed ; nor to 

 the state of the nerve, for it is still sensible to light ; and yet the patient cannot 

 see objects distinctly ; and it is not an uncommon circumstance, even when the 

 operation succeeds, that the sight is slowly and gradually recovered. Instances 

 have occurred, Mr. Bell * observes, of the sight becoming gradually better for 

 several months after the operation. When we have been long out of the habit 

 of combining our muscles in almost any one action of life, as walking, dancing, 

 or playing on a musical instrument, we in a great measure lose the combination, 

 and find a difficulty in recovering it, in proportion to the length of time we had 

 been deprived of it ; but the individual action of each muscle remains as before. 

 Thus probably with the muscles of the eye. A variety of facts of a similar 

 nature must present themselves to every person conversant in the science of optics, 

 which may admit of a similar explanation. 



I have thus endeavoured, first, to point out the limited action of the iris, and 

 of consequence the insufficiency of this action for explaining vision. 2dly, to 

 prove that the lens possesses no power of changing its form to the different dis- 

 tances of objects. Sdly, that to see objects at different distances, corresponding 

 changes of distance should be produced between the retina and the anterior part 

 of the eye, as also in the refracting powers of the media through which the rays 

 of light are to pass. And 4thly, that the combined action of the external muscles 

 is not only capable of producing these effects, but that from their situation and 

 structure they are also peculiarly adapted to produce them. Is it not then con- 

 sistent with every principle in the economy of nature and of philosophy, seeing 

 the imperfections of the principles which have hitherto been employed in explain- 

 ing the phenomena in question, to adopt the one before us, till, agreeable to one 

 of the established rules in philosophizing, other phenomena occur, by which it 

 may be rendered either more general, or liable to objections ? 



I have now finished what was proposed. I have declined entering into an ex- 

 tensive view of the structure of the eye, or any of the general principles of optics, 

 as those subjects have been more ably treated in the works already cited, and thus 

 would certainly have destroyed every claim to attention, which these few pages in 

 their present form may possibly possess ; and if I should be so fortunate as to 

 succeed in establishing the principle I have proposed, for explaining the pheno- 



• See his System of Surgeiy. 



