1886.] Light reflected at nearly Perpendicular Incidence. 291 



Silver-on- Glass Speculum. 



This was the silver side of the same glass as the last, polished with 

 wash leather and a little rouge. The milky film was not perfectly 

 removed. Four observations, not over concordant, probably in con- 

 sequence of variation of reflecting power at different parts of the 

 surface, gave — 



Lord Eayleigh. Mrs. Sidgwick. Mr. Gordon. 



Aug. 14. . 0-902 .... Aug. 16. . 0-933 .... Aug. 14. . 0'920 

 „ 16.. 0-895 



Mean. . 0-912. 



The surface was then repolished and remounted with the following 

 results : — 



Lord Eayleigh. 



Aug. 18. . . . 0-950 

 „ 19.,.. 0-938 



Mean, 



0-944 



Mr. Grordon. 

 Aug. 18. . . . 0-952 

 „ 19.... 0-911 

 „ 21.... 0-938 



Mean. 



0-934 



Mean. . 0-939. 



The increase in efficiency may have been due to a more careful 

 selection of the best polished central part as much as to actual improve- 

 ment in the polish of the speculum as a whole. The transmission of 

 the disk used with this surface is 0'9105. 



Sir John Conroy* found an even higher number (0*975) as the 

 reflecting power of silver films for light polarised in the plane of 

 reflection, and incident at 30°. 



Mirror of Black Glass. 



A plate of opaque glass has the advantage that the influence of 

 the hinder surface is eliminated without more ado ; but, on the other 

 hand, it lends itself less readily to determinations of index, The 

 following results were obtained with such a plate: — 



July 29. 



„ 30. 



„ 31. 

 Aug. 2. . 



„ 3.. 



Mean 



Mrs. Sidgwick. 

 0-0580 

 0-0581 

 0-0583 

 0-0574 

 0-0581 



Lord Eayleigh 



0-0570 

 0-0572 

 0-0578 

 0-0577 



0-0580 



Final mean. . 0-0577. 



0-0574 



* ' Eoy. Soc. Proc.,' vol. 37, 1884, p. 33. 



