14 Mr. W. H. M. Christie on the [Mar. 1, 



flint, the refractive indices for A andH being ^=1*700 and /i H = 1*770. 

 The refracting angle of the half -prism =\j/'. In this case 



A = — tan \L and — = — tan 

 25 T m 14 



Half-prism of Flint. 



Dispersion and Magnif jing-power. 









A. 



cos tJ/h 



m A. C ° 9 f • 

 COS 



o 



41 35 



39 36 









22 



1 59 



1-24 



1-20 



24 



46 5 



43 47 



2 18 



1-32 



1-27 



26 



50 50 



48 7 



2 43 



1-42 



1-35 



28 



56 7 



52 52 



3 15 



1-57 



1-45 



30 



62 15 



58 13 



4 2 



1-86 



1-64 



32 



69 44 



64 17 



5 27 



2-45 



1-95 



34 



81 40 



71 48 



9 52 



5-72 



2-66 



34=| 





74 20 







3-05 



35 





77 10 







3-69 



35| 





80 45 







5-12 



36 





87 43 







20-20 



Light reflected at emergence. 





Polarized 











Total. 







in plane 



in perpendicular 







of incidence. 



plane. 





o 



per cent. 



per cent. 



per cent. 



30 





[ H 28-5 



0-0 



14-3 





A 22-4 



o-o 



11-2 



32 





r H39-l 



2-5 



20-8 





A 28-9 



0-5 



14-7 



34 





1 H 67-1 



27-7 



47-4 





A 40-5 



4-8 



22-7 



34| 



A 45-9 



8-3 



27-1 



35 



A 52-2 



13-3 



32-8 



35i 



A 62-6 



24-8 



43-7 



36 



A 94-4 



71-6 



83-0 



The above numbers express the percentage of light lost by reflexion, 

 computed by Fresnel's formulae for H and A respectively. 



These Tables will enable us readily to compare prisms of different angles 

 in the case of this particular quality of glass, which has been selected as 

 giving large dispersion with moderate absorption. Let us take two 

 examples for comparison with direct-vision prisms. 



