OBSERVATIONS AND EXPERIMENTS ON VISION. 331 



effect, either wholly, or very nearly so, is manifest, from 

 the necessity even young persons are under, who have lost 

 it, of using glasses of very different convexities for near 

 and remote objects. But in what way this important office 

 is performed by it seems still unknown. The learned Dr. though it has 

 Young, indeed, as well as others before him, has supposed, proved that 

 that the crystalline has the power of altering its figure; but this can alter 

 the proofs hitherto given in favour of this opinion appear* *"'^^* 

 very defective. In 1794, I attempted to submit its justness 

 to the test of direct experiments, by applying to the crystal- 

 lines of oxen, which had been felled from thirty seconds to 

 a minute before, chemical and mechanical stimuli, and those 

 of galvanism and electricity ; but in no instance was any al- 

 teration of figure, or other indication of muscular power, 

 observed. All of these stimuli were applied to the crystal, 

 line while it was surrounded by air, and some of them while 

 it was covered with warm water. Last summer, after I 

 knew that men lose, from increase of years, the faculty of 

 altering the refractive state of the eye, I thought it possible, 

 that the oxen on which I had made the experiments were too 

 old for them. I therefore repeated most of them on the 

 crystallines of a calf and a Iamb; but still no motion was 

 to be seen. Dr. Young has made similar experiments with 

 a similar event ; but he thinks, that no argument can hence 

 be derived against his opinion, as neither can motion be ex- 

 cited in the uvea, by any artificial stimulus. In the first 

 place, however, it is not agreeable to just reasoning, to re- 

 gard an unknown thing as an exception to a general rule, 

 rather than as an example of it; in the second, the motions 

 of the uvea are involuntary, whereas the adaptation of the eye 

 is, in part at least, under the command of the will; and in 

 the third, the crystalline seems very unfit for performing the 

 motions which he assigns to it; for if its figure be altered 

 out of the body, by external force, it does not restore it- 

 self, but retains the shape which has been given to it, like 

 a piece of dough, or soft clay. Possibly farther experi- 

 ments with belladonna may contribute to remove the ob- 

 scurity which at present surrounds this subject. 



II. Method 



