234 



LECT. 

 VII. 



Lenses. 



FERGUSON'S LECTURES. 

 B C J> E . F G 



H 



1. A plane glass, which is flat on both sides, and of 

 equal thickness in all its parts, as A. 



2. A plano-convex, which is flat on one side, and con- 

 vex on the other, as B. 



3. A double convex, which is convex on both sides, 

 as C. 



4. A plano-concave, which is flat on one side, and 

 concave on the other, as D. 



5. A double concave, which is concave on both sides, 

 as E. 



6. A meniscus, which is concave on one side, and con- 

 vex on the other, as F. 



. 7. A flat plano-convex, whose convex side is ground 

 into several little flat surfaces, as G. 



8. A prism, which has three flat sides, and when 

 viewed endwise, appears like an equilateral triangle, 

 as H. 



Glasses ground into any of the shapes B, C, D, E, P, 

 are generally called lenses. 



A right line L I K, going perpendicularly through 

 the middle of a lens, is called the axis of the lens. 



A ray of light G h, falling 

 perpendicularly on a plane glass, 

 E F, will pass through the glass 

 in the same direction h i, and go 

 out of it into the air in the same 

 right course z H. 



A ray of light A B, falling ob- 

 liquely on a plane glass, will go out of the glass in the 

 same direction, but not in the same right line ; for in 

 touching the glass, it will be refracted in the line B C, 



