196 



Sir J. Conroy. 



[Jan. 10, 



of the mirror had been diminished by the formation and removal of 

 this film on several occasions. 



Light reflected, 

 per cent. 



62- 42 



63- 04 

 63-30 



Mean 62*92 



The mean of these three observations agrees very fairly with the 

 value deduced from the observations with polarised light, and it 

 therefore seems probable that the differences between the numbers 

 contained in the sixth and seventh columns of Table YIII are due to 

 an alteration in the surface of the mirror, analogous possibly to the 

 surface changes that Seebeck found with freshly polished transparent 

 bodies. 



The values obtained for the principal incidence, and principal 

 azimuth, with the freshly polished mirror, and with the same mirror 

 at the conclusion of the experiments with polarised light, differ but 

 little from each other, as is shown in Table VII. The first four 

 observations with the speculum metal mirror were made at the end of 

 the experiments, and their means 75° 35' and 33° 12' agree very 

 closely with the means of the last two observations 75° 29' and 32° 57' 

 made with the freshly polished mirror. 



The amount of light which, according to Cauchy's theory, ought to 

 have been reflected by the mirrors was calculated out by the 

 formulae — 



j 2 _ 0° + cos 2 i — 20 cos e cos i -j- 2 _ # 2 cos 2 i + 1 — 20 cos e cos i 

 2 + cos 2 i + 20 cos e cos i 2 cos 2 i + 1 + 26 cos e cos i 



and the results set forth in Tables X and XI. 



Table X. — Amount of Light reflected by Steel Mirror. 





Observed. 



Calculated. 



J 2 . 



12. 



J 2 . 



I 2 . 



o 



30 



60-70 



50-19 



63-17 



54-95 



40 



64-21 



46 -28 



66-44 



51-31 



50 



68-52 



40 -98 



70-80 



42-09 



60 



74-42 



34-78 



76-72 



39 -24 



65 



77 '37 



30-03 



79-52 



35-32 



70 



82-26 



26-54 



83-04 



31 -62 



75 



86 01 



24-73 



86 -85 



29 -46 



80 



87 -87 



26-60 



90-97 



32-39 



