330 ORSERVATIONS AND EXrEKIMENT* ON VMIOK, 



pathy betweea the pupils. For when the pupil of one eye 



becomes dilated from the application of belladonna, the 



pupil of the other, so far from dilating, becomes smaller. 



Pupil of one It follows, therefore, that the size of the pupil is dependant, 



eye affected by , , ... . ^ ,. , '., 



the impression Dot only on the impression of light on the retina of its own 

 oflightonthe eye, but on that also which is made on the retina of the 

 other; and that the moTing of the two together, which for 

 the most part takes place, is only an accidental consequence 

 of the fact which I have mentioned. 

 Capacity of the 2. As the action of the external muscles of the eye has 

 eye not owing ]^qq^ frequently resorted to, for an explanation of its capa- 

 muscles; city to see objects perfectly at different distances, I request- 



ed Dr. Cutting to attend to this matter. He accordingly as. 

 certained, while his eye was in its natural state, the distance 

 from his face of the nearest point, at which he could make 

 the two optic axes meet, this being the greatest trial of 

 strength, to which those muscles can be exposed. Shortly 

 after, he repeated the experiments, while, in consequence 

 of the application of belladonna, he was without the power 

 of adapting his eye to different distances, and found, that 

 the strength of those muscles was not diminished. It fol- 

 lows, therefore, not only that the external muscles have 

 little or no concern in fitting the eye to see distinctly at dif- 

 ferent distances, but that the same is true with respect to 

 the cornea; as we cannot suppose, that its mechanical pro- 

 perties were altered by the belladonna, or at least, that it 

 became more inflexible from the application to it of the juice 

 of that herb. I had before made a similar experiment on 

 myself, by comparing what had been the strength of the ex- 

 ternal muscles of my eyes twenty years ago *, with what it 

 •was after I had lost the power of altering their refractive 

 state; but though I found no difference, yet, as their coats 

 might have in the mean time become more rigid, I thought 

 it right to have the experiment repeated, in a manner to 

 which no objection could be taken, 

 but apparently ^^^ ^^^Y other part of the eye, or its appendages, which 

 to the crystal- remains for enabling us to see equally well at very different 

 distances, is the crystalline; and that it does produce this 



* Essay on Single Vision^ Sec. p. 136. 



effects 



