34 



Sir J. Conroy. 



[Feb. 15, 



At the angle of polarisation 



u=2p, (U=sinB tanB), 



when /3=the principal azimuth and B the principal incidence. 



The forms which have been given to these expressions by MM. de 

 la Provostaye and Desains ("Ann. de Chim." (3), 30, p. 279), are 

 more convenient for calculation, and therefore were used. 



jo 2 + cos 2 i—20 cos e cos?' 



2 + cos 2 i + 20 cos e cos i 



jo _ 2 cos 2 i + 1 — 20 cos 6 cos i 

 0% cos 2 i + 1 + 20 cos e cos i 



These authors remark in their paper that it is generally sufficient 

 to take e=u, and therefore 0=U,* and this was done in calculating out 

 the intensities. The incident light having been unpolarised, half the 

 sum of the intensities of the light polarised in, and perpendicularly to 

 the plane of incidence {i.e., \ (J 2 + I 2 )} was taken as the theoretical 

 intensity of the reflected light. 



Amount of Light Reflected by the four Mirrors. 





Silver 





Steel. 



Angle of 













Incidence. 















Observed. 





Calculated. 



Observed. 



Calculated. 



o 



20 



70-06 





80-97 



55 -39 



59-19 



40 



70-87 





80 -84 



55 -62 



58 -92 



60 



74-19 





80 -24 



57-63 



57 -66 



80 



81 -19 





83 -68 



63 -56 



60-71 





Tin. 





Speculum Metal. 



20 



40 -28 





60-55 



66 -88 



66 85 



40 



44-11 





60 -41 



67-26 



66 -62 



60 



50-60 





60 -01 



66-32 



65-64 



80 



65 -08 





66 -98 



70-17 



69-60 



The table shows that with the speculum metal mirror the observed 

 and calculated intensities agree fairly well, but that such is not the 



* According to Lundquist (" Pogg. Ann.," 152, p. 410), Jamin himself appears 

 to liave done so ; in this case the formulae (1) for determining (p and x become re- 

 spectively — ■ 



cot <p = cos 2/3 sin ( 2 arc tan — SSl^ — \ , 

 \ sur B cos if 



cot x = cos 2/3 sin ^2 arc tan 



cos B cos i 

 sin- B , 



