10 ELEMENTARY PRINCIPLES < >F MICROSCOPICAL OPTICS 



The values of the same lines for the Hint idass are as follows : 

 I). 1-7174=,,: F. 1-7:US'.)= M ' : ('. l-710r>5=/'. 



- M 



1-7:UH'.> -- 1-7 1 (I");') -0-2434 

 = -- 



, t - i -1-71741 



So the dispersive power of the Ilint between the lines C and F is 

 slightly more than twice that of the crown for the same region of 

 t he spec) rum. In t he a bo\ e formula the expression p' ^u" is usually 



written c 



? 

 in full it is therefore w = 



- 



I Caving thus t raced a rav 

 experimentally through a 

 prism. our next step is to sho^n 



that a cuiii-t'.i- It-it* i.-- nnlti n 

 en rri'il Jnrni <>j' 1irn xtti-li jH-ixm.^ 

 with their liases in contact. 

 as is shown in A. tiy. (5. 

 where the curved line shou.- 

 the lenticular character and 

 the shaded elements 1 he two 

 prisms. A concave lens is in 

 effect two prisms reversed. 

 that is. with their apices in 

 contact, as in l>. til-;, li. \\here. 

 a-ain, the cnr\cd line shows 

 t he form of the lens and the 



Pin. (',. Convex and concave FIG. 7. Proof that a lens maybe considered 



lenses are related to the as an assemblage of prisms. (From tin' 



I rism. Fun-rs ui' Nature.') 



shaded parts its relation to a pair of prisms. The fact that a lens is. 

 in effect, as such, but an assemblage of superposed prisms is seen in 

 fig. 7, the refracting angle of the prism being more acute as the 

 principal axis is approached, and the deviation being greater as the 

 angle is more obtuse. 



In fig. 8 letOP be the axis in each case; then, from \\hai we 

 have seen, it is manifest that rays parallel to the axis falling on the 

 prisms with their bases in contact and acting like a convex lens will 

 be refracted towards the axis U P. Hut in the other case, uhere 

 the prisms have their apices together, as in lig. !, act in- as a con 

 cave lens, the light is refracted away from the axis < > P. 



