24 



Mr. W. H. M. Christie on the 



[Mar. 1, 



This Table gives the angles of refraction and emergence and the 

 deviations at the two refractions (D", D'"), and also the magnifying-power, 

 purity, and dispersion for various direct-vision half -prisms, a being the 

 angle of the flint and /3 the angle of the crown. 



The light lost by reflexion at the refraction from flint to crown is as 

 follows : — 



Light lost by Eeflexion. 





Polarized 











Total. 







in plane 

 of incidence. 



in perpendicular 

 plane. 





o 



54 



per cent. 



per cent. 



per cent. 



12-1 



4-2 



8-2 



56 



19-1 



9-2 



14-2 



56J 



22-0 



11-5 



16-8 



57 



25-4 



14-5 



20-0 



57i 



30-4 



18-9 



24-7 



58 



37-7 



25-7 



31-7 



58|- 



49-2 



37-5 



43-4 



59 



79-1 



72-9 



76-0 



As the amount of light lost in different combinations of prisms requires 

 careful consideration, I have by the help of the preceding Tables calcu- 

 lated the loss in passing through trains of half -prisms] having respec- 

 tively angles of 57° for the flint, with 110° for the crown, and 58^° for 

 the flint, with 120° for the crown. These particular forms of half-prism 

 (which I will call A and B) have been used in two experimental spectro- 

 scopes to be described shortly, and I have therefore selected them as 

 examples. 



Light lost by Eeflexion. 





Polarized. 









in plane 

 of incidence. 



in perpen- 

 dicular 

 plane. 



Total. 



Dispersion. 



One half-prism A . . 

 Two half-prisms A . 

 Three „ „ . . 

 Four „ „ . . 



per cent. 

 40-9 

 65-1 

 79-4 

 87-8 



per cent. 

 21-2 

 37-9 

 51-1 

 61-4 



per cent. 

 31-0 

 51-5 

 65-2 

 74-6 



19-3 

 73-0 

 276-0 

 1043-0 



One half -prism B . . 

 Two half -prisms B . 



61-7 

 85-3 



42-5 

 66-9 



52-1 

 76-1 



45-5 

 380-0 



It will be seen that for equal dispersion the loss of light is slightly 

 greater for a train of half -prisms B ; but this is fully compensated for by 

 the diminished absorption, as fewer prisms will be required. 



