m 



Experiments 



on an old 



eye 



wanting the 

 crystalline in- 

 conclusive. 



Experiments 

 with bella- 

 donna. 



OBSERVATIONS AND EXPERIMENTS ON VISION. 



taocc of seven inches from my eye, the other extremity of 

 vay ancient range of perfect vision, I was now obliged to 

 employ a glass having a focus of only six inches. I regret 

 much, that I had not made such experiments frequently be- 

 fore, as I think it very probable, that I should have found 

 a period in the progress of my vision to its present state, 

 in which my capacity of seeing distant objects was the same 

 as in my youth, and when therefore the whole of my im- 

 perfect vision of near objects would have been owing to a 

 loss of the muscular powers of my eye. 



As there can be no good reason for supposing, that the 

 changes which have occurred in my eyes are different from 

 those, which the eyes of by far the greater number of per- 

 sons, who are not short-sighted, undergo at the approach 

 of old age, it is evident, that the experiments of Dr. 

 Young * on the eye of Hanson, whom the learned author 

 considered as a very fair subject for such trials, furnish no 

 proof, that the want of the crystalline lens disables a per- 

 son from having perfect vision at different distances ; for as 

 Hanson was sixty three years old, it is highly probable that 

 the results of the experiments would have been exactly the 

 same, if he had still possessed that part of his eye. 



III. Having discovered, that ray own eyes were unfit for 

 the experiments, which I wished to be made with bella- 

 donna, I instructed an ingenious young physician, Dr. 

 Cutting, from the island of Barbadoes, and now residing 

 there, in the manner elsewhere described by me +, of as- 

 certaining his range of perfect vision by means of luminous 

 points. This he found, in consequence, to begin, with re- 

 spect to his left eye, at the distance of six inches, and not 

 to terminate at the distance of eight feet ; beyond which he 

 could not see clearly the object, with which he had hitherto 

 made his experiments, the image of the flame of a candle 

 in the bulb of a small thermometer. The flame of a lamp, 

 distant about sixty yards, gave a faint indication of its rays 

 meeting before they fell upon the retina ; the rays from a 

 star had very evidently their focus a little before that mem- 



• Phil. Trans. 1801, p. 66 : see Journal 4to series, vol. v, 

 f Essay on Single Vision, &c. p. 110. 



brane. 



