324 WELLS'S NATURAL PHILOSOPHY. 



each other, the image of the arrow appears inverted. The central ray of light 

 proceeding from the object in tlie direction of tlie axis G, and falling perpen- 

 dicularly upon the surface of tlie lens, undergoes no refraction, but continues 

 on in a direct course. 



The images thus formed hy convex lenses maybe rendered 

 ageTforn^dby ''^'sible by being received upon white screens, or any suitable 

 convex lenses objects, or directly by the eye, when placed in a proper posi- 

 be made visi- ^. , • ii 



jjjgj tion to receive the rays. 



When, by the employment of the convex lens as a burning- 

 glass, we concentrate on any suitable surface, the sun's rays to a focus, the little 

 luminoiis spot, or circle formed, is really an image, or picture of the sun 

 itself. 



Why are con- 683. Convex leiises, as ordinarily used, are 

 irMa^Tfy^ng Called Hiagnifying-glasses, because tbey in- 

 Giasses? creasG the apparent size of tbe objects seen 



througli them. 



The reason of this is, that the lens so alters, 

 convex lens by rcfraction, the direction of tbe rays of light 

 magni y procccding froiu an object, that tliey enter tbe 



eye as if they came from points more distant from each 

 other than is actually the case, and hence the object ap- 

 pears larger, or magnified. 



On the contrary, the concave lens, which 



Why does a y ■ nn t 



corcave lens produces an exactly opposite eiiect upon the 



diminish the •' n i- i i • n j ' . 



apparent size lavs 01 lifflit, causcs the Huaire ot an obiect 



of an object? ,i i •- x fl 



seen through it to appear smaller. 



On the same principles also, concave mirrors magnify, and convex mirrors 

 diminish the images of objects reflected from their surfaces. 



Hence the magnifying or diminishing power 



■What is said of .- , , ■ n i , i 



the magnifymg 01 Icuscs IS uot, as IS oitcu popularly supposed, 



or diminishing , , ' ,. p -l t r 



power of lenses? ctue merely to the pecuhar nature oi the glass ot 

 which they are made, but to the figure of their 

 surfaces. 



The double convex lens, inclosed in a convenient setting of metal or horn, 

 if extensively employed by watch-makers, engravers, etc., with whom it 

 passes under the general name of lens. 



How may con. 684. In additiou to the efiect which convex 

 derdishT^ob- lenses produce by magnifying the images of 

 jects visible? objccts, they are also capable of rendering 

 distant objects visible which would be invisible to the 



