Rcfrangibility of Light. 161 



represent an arrow, which would be seen by 

 the eye, if no glass were between, by the con- 

 vergent rays, ca and db ; but if the concave lens 

 D be interposed between the object and the eye, 

 the line ac will be bent towards g*, and the line 

 bd will be bent towards k 9 and consequently both 

 will be useless, as they do not enter the eye. The 

 object then will be seen by other lines, such as ao 

 and 6r, which, on entering the glass, will be 

 refracted, and bent in the directions oc and rd. 

 According to the rule just now laid down, there- 

 fore, every object is seen along the line which 

 enters the eye last. The arrow is seen according 

 to the angle or, which is much smaller than the 

 angle db ; consequently it will appear considerably 

 diminished, and at the distance of nm. 



The spectacles which are used by near or short- 

 sighted persons consist of concave lenses; for 

 the reason of short sight is, that, the form of the 

 eye being too convex, the rays are made to con- 

 verge before they reach the optic nerve ; and there- 

 fore the concave glass, causing a little divergence, 

 .assists this defeat of sight. But this matter will 

 be still further explained when we treat of vision. 



The meniscus, No. 6, is properly like the 

 crystal of a common watch, and it neither mag- 

 nifies nor diminishes. Sometimes, however, it is 

 made in the form of a crescent ; that is, thickest 

 in the middle; and in that case it acts like a 

 double convex lens. 



It is evident that all lenses, as to their surfaces,, 



