VOL. LXVIII.] PHILOSOPHICAL TRANSACTIONS. 2Qy 



that Dr. D. could not by any means induce him to bend the axes of both his 

 eyes on the same object not even for a moment. A gnomon of thin brass was 

 made to stand over his nose, with a half circle of the same metal to go round 

 his temples ; these were covered with black silk, and by means of a buckle 

 behind his head, and a cross piece over the crown of his head, this gnomon was 

 managed so as to be worn without any inconvenience, and projected before his 

 nose about 2-^ inches. By the use of this gnomon he soon found it less incon- 

 venient to view all oblique objects with the eye next to them, instead of the 

 eye opposite to them. After this habit was weakened by a week's use of the 

 gnomon. 2 bits of wood, about the size of a goose-quill, were blackened, all 

 except 4 of an inch at their summits; these were frequently presented for him 

 to look at, one being held on one side the extremity of his black gnomon, and 

 the other on the other side of it. As he viewed these they were gradually 

 brought forwards beyond the gnomon, and then one was concealed behind the 

 other : by these means, in another week, he could bend both his eyes on the 

 same object for half a minute together. By the practice of this exercise before 

 a glass, almost every hour in the day, he became in another week able to read 

 for a minute together with his eyes both directed on the same objects; and 

 Dr. D. had no doubt, if he had patience enough to persevere in these efforts, 

 but he would in the course of some months overcome this unsightly habit. 



Dr. D. concludes the account of this case by adding, that all the other 

 squinting people he had had occasion to attend to, had one eye much less 

 perfect than the other, according to the observations of Mr. BufFon and Dr. 

 Reid. These patients, where the diseased eye is not too bad, are certainly 

 curable by covering the best eye many hours in a day ; as, by a more frequent 

 use of the weak eye, it not only acquires a habit of turning to the objects 

 which the patient wishes to see, but gains at the same time a more distinct 

 vision ; and the better eye at the same time seems to lose somewhat in both these 

 respects, which also facilitates the cure. 



This evinces the absurdity of the practice of prohibiting those who have weak 

 eyes from using them ; since the eye, as well as every other part of the body, 

 acquires strength from that degree of exercise which is not accompanied with 

 pain or fatigue ; and he was induced to believe, that the most general cause of 

 squinting in children originates from the custom of covering the weak eye, 

 which has been diseased by any accidental cause, before the habit of observing 

 objects with both eyes was perfectly established. 



The facility with which master Sandford received the images of oblique objects 

 on the insensible part of the retina of one eye, while he viewed them with the 

 other, induced Dr. D. to observe the size of this insensible spot, and to endea- 

 vour to ascertain the cause of it. There was formerly a dispute among philoso- 



« a '2 



