4 



816 THE SENSE OF SI 



is to be expected, since light in passing from a rare into a dense medium 

 suffers a retardation, and this diminution in its velocity affects its 

 component rays differently, i.e., those at the red end, with long wave- 

 lengths, are refracted the least and those at the violet end, with short 

 wave-lengths, the most. Inasmuch as a lens is composed of a series 

 of prisms — and prisms split the white light in accordance with the 

 unequal refrangibihty of its simple color components — a spectrum must 

 result (Fig. 424). Thus, white Hght, when passed through the edge 

 of a biconvex lens, is dispersed so that "its violet rays are brought 

 to a focus (7) in front of its red rays {R), while the foci of its orange, 

 yellow, green, blue and indigo are situated in between these two ex- 

 treme's. This condition which is called chromatic 'aberration, is also 

 pre'^eat ini the lens of our eye, but cannot seriously interfere with the 

 fo^- :ii ; -^f the image, because the iris does not permit the rays of 



-\--. 



Fig. 424. — Diagram lLi.T7SrE.vriNa Cu< 

 The iria being retractel, tho rays of white light ni . . 

 the lens are split into their spectral componentSi Tbi vir.i L - 

 the leuB than the red. ' 



light to pass through its more poony iciru-cimg [:•" ■;• on, 



By analogy, it may be conc^udeo that the jaydrmiAr . *>- 



matically aberrated in '.ages, because the e;%e of it- 



to the entering beam of light. 'In artiiicial U n^. 



often overcome by combining crown glas^with 



as the di^persi ve power of f ' 



disf)ersion may ih'-rr- 



their trial refrai ; 



Dollar i in the .... .^ .......,,, id 



beini^ le term appa<.; )n of the refractioi 



^'i' lecompoBition inu; - 



t well-known fact ti at a .ed and vioK 



pLi. the same distance m front oi do, the former appears 



or f>rorr; ' A ! a * I seems nearer tO m Clearly, this --^ ^ 



-1 '-• ' j' ^ ; 'r>nse since the rjd rays possess 



