80 PHILOSOPHICAL TRANSACTIONS. [ANNO 17Qf. 



states of the eye at present under consideration. The cases brought forward with 

 this view, are rather to be considered as the detail of so many experiments made 

 in the investigation of the diseases, than as histories of particular patients. When 

 muscles are strained or over- fatigued, to put them in an easy state, and confine them 

 from motion, is the first object of attention ; and this practice is no less applicable 

 to the muscles of the eye, than to those of other parts. 



\ Of squinting. — Whenever the motions of the 2 eyes differ from each other, 

 whether in a less degree, so as to produce double vision, or in a greater, turning 

 one eye entirely from the object, the disease has been called squinting. What I 

 mean at present to consider under this head is, where the deviation of one of the 

 eyes from the axis of vision is greater than that by which objects are made to 

 appear double ; so that in this view, double vision is an intermediate state between 

 single vision with both eyes, and squinting. Squinting has been very generally 

 believed to arise entirely from an inability in the muscles to direct the eye properly 

 to the object. There is however probably no original defect in the muscles ; cer- 

 tainly none sufficient to sanction such an opinion ; since the muscles of a squinting 

 eye have the power of giving it any direction, but cannot do it without some de- 

 gree of effort. The defect therefore appears to be principally in the eye itself, 

 which is too imperfect to assist the other in producing distinct vision. From this 

 imperfection, the muscles have not the same guide to direct them as those of the 

 other eye ; and therefore, though perfectly formed, cannot make their actions 

 exactly correspond with them. 



In a squinting person, both eyes certainly do not see the object looked at. This 

 is evident to a by-stander, who is able to determine, that the direction of one of 

 the eyes differs so much from that of the other, that it is impossible for the rays 

 of light from any object to fall on the retinas of both ; and -therefore, that one eye 

 does not see the object. The same thing may be proved in another way : for since 

 a small deviation in the direction of either eye from the axis of vision, produces 

 double vision, any greater deviation must have the same effect, only increasing the 

 distance between the 2 images, till it becomes so great that one eye is directed to 

 the object. In squinting there is evidently a greater deviation from the axis of 

 vision than in double vision, and the object does not appear double ; it is therefore 

 not seen by both eyes. 



The circumstance of those who squint having an imperfect eye, is corroborated by 

 all the well authenticated observations which have been made on persons who have 

 a confirmed squint, which all agree in stating, that one of the eyes is too imperfect 

 to see distinctly. From these observations, it would be natural to suppose that 

 the loss of sight in one eye, should produce the appearance of squinting, which is 

 by no means the case ; for when that happens, the motions of the 2 eyes continue 

 to correspond, though not exactly ; but the deviation not equal to that which is 

 met with in squinting ; it is nearer to that which occurs in double vision. 



The reason why the imperfect eye of a squinting person is directed from the 



