78 PHILOSOPHICAL TRANSACTIONS. [ANNO 1797. 



ticed in a former lecture, in treating of the adjustment of the eye. At first it 

 appeared difficult to account for the double vision, particularly as the 2 images were 

 entirely separate from each other. It could not arise from the absence of the lens, 

 as that would not alter the situation of the images on the retina : and the 2 images 

 being of different dimensions on similar parts of the retina, would appear to be 

 one before the other. As the operation of extracting the lens in no respect affects 

 the muscles of the eye, the action of the muscles would be the same as before, 

 and therefore could not contribute to produce this effect. The double vision in this 

 instance appears to arise from the cornea of the eye which had undergone the 

 operation being rendered flatter than the other, and giving a different direction to 

 the rays of light, so as to form an image on a part of the retina not corresponding 

 with the part impressed in the other eye. 



If the crystalline lens be extracted from both eyes, and the person applies a 

 convex glass to one eye only, and looks at an object, it will appear double ; but if 

 the convex glass is moved in different directions before the cornea, there will be 

 found one situation in which it makes the object single. In this instance the cor- 

 neas and muscles of the 2 eyes are under exactly the same circumstances ; and 

 when the centre of the convex glass is directly in the axis of vision, the image on 

 the retina of that eye is formed on parts that correspond with those impressed in 

 the other ; but whenever the centre of the convex glass is out of the axis of vision 

 this does not take place, and the object appears double. The experiments of 

 which these observations are the result, were made on the eyes of a lady who had 

 lost the sight of both, by opacities in the crystalline lenses ; but by submitting 

 to have the lenses extracted she recovered her sight, and had afterwards an un- 

 common degree of distinct vision ; which made her a very favourable subject for 

 experiments of this kind. 



Having explained the 2 different modes by which double vision may take place 

 in consequence of operations that render the refracting media of the eye imperfect, 

 I shall now consider it when produced by a morbid action of the muscles. Several 

 cases of this kind have come within my own knowledge, and I am induced to dwell 

 on the subject, because some of them had been considered as arising from a defect 

 in the organ, and erroneously treated. The fact has been long established by 

 philosophers that a defect in the muscles may produce such a disease ; but as other 

 causes may also do the same, I believe that such a defect has not been practically 

 considered, as one of the diseases of the eye ; certainly not as a very common one, 

 which undoubtedly it will be found. 



The first case of this kind which led me to pay attention to the subject, was that 

 of a friend, a lieutenant-colonel of engineers, who was in perfect health, shooting 

 moor-game on his own estate in Scotland. He was very much surprized towards 

 the evening of a fatiguing day's sport, to find all at once that every thing appeared 

 double; his gun, his horse, and the road, were all double. This appearance 

 distressed him exceedingly, and he became alarmed lest he should not find his way 



