LENSES. 



337 



and to be concave when the opposite is the case. Thus A, B, C 

 (fig. 182) represent three forms of convex lens, and D, E, F three 

 forms of concave lens, respectively termed double convex and piano 

 convex (or convexo plane, according as the curved or plane surface 

 meets the rays of light first), and concavo convex (or convexo 



B C D E 



Fig. 182. Convex and Concave Lenses. 



concave); and double concave, piano concave (or concavo plane), 

 and concavo convex (or convexo concave). To distinguish the 

 two forms of concavo convex lenses, the first is sometimes called a 

 meniscus, and the terms concavo convex and convexo concave 

 reserved for the other ; sometimes the two kinds are distinguished 

 as converging and diverging respectively. 



When rays of light, SA, SB (fig. 183), from a source at so great 

 a distance that the rays are practically parallel, meet any kind of 

 convex lens, they are refracted so as to meet one another at a focus, 

 F, just as with a concave mirror ; whilst if the lens be concave 

 the rays of light diverge after passing through, as if they had em- 

 anated from a virtual focus, as with a convex mirror (fig. 184). 

 The refrangibility of differently coloured rays not being the same, 



Fig. 183. Real Focus of Convex 

 Lens. 



Fig. 184. Virtual Focus of 

 Concave Lens. 



it results that the more refrangible rays are brought to a focus by 

 a convex lens a little nearer to the lens than the less refrangible ones, 

 so that the position of the focus is not quite the same for all colours. 

 This difference is termed chromatic aberration, and its existence 

 produces a very inconvenient result with many kinds of optical in- 

 struments, viz., that images formed by means of lenses are apt to 



