318 Lord Rayleigh on the Intensity of Light reflected from 



t denoting the temperature in Centigrade degrees. Applied 

 to 18° and to \= 5620, this gives 



/u= 1-333951, 



whence 



(/*-l) 2 /(/* + l) 2 =*02047. 



The reflexion actually found is accordingly about 1J per 

 cent, greater than that given by Fresnel's formulae. 



In order to estimate the effect, according to the formula, of 

 a change in index, we may use 



SR__ 4fy* 

 R ~ v?-V 

 or, in the case of water, 



SR/R— 5S//, nearly. 



To cause a variation of li per cent, in the reflexion, S/j, would 

 have to be '003, and to cause 6 per cent. Sfi would have to be 

 •012. The latter exceeds the variation of /a in passing be- 

 tween the lines D and H. 



The agreement with Fresnel's formulae is thus pretty good, 

 but the question arises whether it ought not to be better. 

 Apart from a priori ideas as to the result to be expected, I 

 should have estimated the errors of experiment as not likely 

 to exceed one half per cent., and certainly no straining of 

 judgment in respect of the photometric pictures would bring 

 about agreement. On the other hand, it must be remembered 

 that one per cent, is not a large error in photometry, and 

 that in the present case a one per cent, error in the reflexion 

 is but one in 5000 reckoned as a fraction of the incident light. 

 While, therefore, the disagreement may be real, it is too 

 small a foundation upon which to build with any confidence. 



It only remains to record the results of some observations 

 upon the reflexion from mercury. In these experiments the 

 revolving disk was dispensed with, and the photographs were 

 taken upon Edwards' plates through yellow glass. The angle 

 of incidence and all the other arrangements remained as 

 before. In order to obtain a balance it appeared that the 

 direct light required to be screened for 64 seconds out of 

 120 seconds. The reflexion is accordingly 



56 /104^3\« 



120 V 82-21 / ~ 

 The mercury was of good quality, and was filtered into a 

 glass vessel just before use. The level was adjusted to be the 

 same as that adopted for the observations upon water. A 



