Water and Mercury at nearly Perpendicular Incidence. 317 



second picture 48 seconds. This means that while in both 

 pictures the exposure for the reflected light was 12 minutes 

 or 720 seconds, the exposures for the direct light were re- 

 spectively 720-2 x 71 = 578 seconds, and 720-2x48 = 624 

 seconds. The water was distilled, and its temperature was 

 17°* 7 C. The examination of the finished pictures showed 

 that the contrast was reversed, so that the total exposure (to 

 the direct light) required for a balance was intermediate be- 

 tween 578 and 624, and, further, that the first mentioned was 

 the nearer to the mark. 



The general conclusion derived from a large number of 

 photographs was that the balance corresponded to a total 

 screening of 121 seconds, viz., to an exposure of 720 — 121 

 = 599 seconds. This is for the direct light, the exposure to 

 the reflected light being always 720 seconds. The ratio of 

 exposures required for a balance is thus 



599 : 720 ; 



and this may be considered to correspond to a temperature 

 of 18° C. 



We can now calculate the observed reflexion for 6^° inci- 

 dence, reckoned as a fraction of the incident fight. We have 



599 J67_ / 104-43y 



720 ' 10800 A 82-21 ) ~ UZU ' b * 



The above relates to the impression upon Edwards' plates 

 after the light had been transmitted through a yellow glass. 

 When Ilford plates were substituted and the yellow glass 

 omitted, the reflexion appeared decidedly more powerful, and 

 the ratio of exposures necessary for a balance was about 

 425 : 480, or 637 : 720. It appears, therefore, that the re- 

 flexion of the light operative in this case is some 6 per cent, 

 more than before, or about *0220 of the incident light. As to 

 a large increase of reflexion there was no doubt ; but, owing 

 perhaps to variations in the quality of the light, the agree- 

 ment between individual results was not so good as before. 



It now remains to calculate the reflexion as given by 

 Fresnel's formulae ; and it appears from the discussion at the 

 commencement of this paper that we may ignore the small 

 angle of incidence (6^°) and take the formula in the simple 

 form given by Young, viz. : — 



R=(,*-1)7( M +1) 2 . 



As to the value of ji for water, Wiillner * gives 



/* x = l-326067--000099* + -30531\- 2 , 



* Pogg. Attn. Bd. cxxxiii. 



