Refrangibility of Light. 153 



on both sides, like the glasses near-sighted peo- 

 ple generally use, as No. 5. Sixthly, it may be 

 concave on one side and convex on the other, 

 like the crystal of a watch, though not in such a 

 degree, as No. 6 ; this is usually called a menis- 

 cus. Seventhly, it may have one side, which 

 must be convex, ground into little facets, like 

 those of some jewels, while the other side is plain. 

 Children know it by the name of a multiplying- 

 glass, as Ncu 7. 



The effects of these different glasses will be 

 easily understood from what has been premised. 

 A ray entering the plain glass, No. 1, will indeed 

 be refracted by the glass, but it will suffer another 

 refraction on going out of it, which will nearly 

 rectify the former; the place of the object will, 

 therefore, as was before stated,, be a little changed, 

 but its figure will remain unaltered. 



If^ again, several parallel rays enter the glass, 

 No. 2, plain on one side and convex on the other, 

 as in figure 50, they will be differently refracted, 

 in proportion to the obliquity with which each 

 of them falls upon the surface. The middle 

 ray, for instance, which passes perpendicularly 

 through, will not be refracted at all, but go on 

 straight forward. All the other rays, howeyer, 

 will suffer refraction. The ray CE., fig. 50, will 

 be refracted upwards to F ; the ray A D will be 

 refracted downwards to the same point. There 

 they will cross, and then go onward, diverging 

 or separating from each other for ever ; that which 



H5 



