OPTICS. &5? 



right eye,) the object will disappear, and seem to 

 be lost; and moving it still farther, it will appear 

 again. Now this place is not at the bottom of the 

 eye, but nearer the nose in both of them ; so that 

 no rays, either parallel or diverging, that come 

 from any object, can fall upon that place in both 

 the eyes; and any object we direct the eyes to, 

 will always be visible, at least to one eye. But 

 the same bright object may be made to disappear 

 to both eyes, by directing the axis of both eyes to 

 a point a little beyond the nose, to be found by 

 trials. 



Dimness of sight generally attends old people; 

 and this may arise from two causes: 1. by the eyes 

 growing flat, and not uniting the rays at the retina, 

 which causes indistinctness of vision ; or 2. by the 

 opacity of the humours of the eye, which in time 

 lose their transparency in some degree; from 

 whence it follows, that a great deal of the light 

 that enters the eye is stopped and lost ; and every 

 object appears faint and dim. 



As the rays of light flowing from an object, and 

 painting its image upon the retina, are the imme- 

 diate causes of seeing; so where there is no light, 

 there can be no vision : consequently, without light, 

 the eye becomes a machine utterly useless. 



OF SPECTACLES. 



We explained before, that if objects are seen 

 through a perfectly flat glass, the rays of light pass 

 through it, and from them to the eye, in a straight 

 direction, and parallel to each other; and, conse- 

 quently, the objects appear very little either dimi- 

 nished or enlarged, or nearer, or farther off, than 

 to the naked eye; but if the glass they are seen 

 through have any degree of convexity, the rays of 



vol. i. s 



