1883.] 



Some Experiments on Metallic Me flection. 



39 



Speculum Metal. Steel. 



58 73 



56 13 44 74 



59-43 44-74 



57-82 44 -20 



Mean 58 "03 44 "42 



With the exception of the first determination made with the 

 speculum metal mirror, which is the mean of eight, each observation 

 is the mean of four readings. 



The principal incidences and azimuths of the mirrors when in 

 water were determined by the same method that had been used for 

 ascertaining the values of these constants in air; the mirrors being 

 surrounded by a cylindrical glass vessel filled with water. 



Speculum Metal. Steel. 



P.I. P.A. P.I. P.A. 



72° 14' 32° 30' .... 72° 37' 27° 41' 



72 15 31 53 .... 72 17 27 14 



Mean. . 72 14 32 11 .... 72 27 27 27 



These values were introduced into Cauchy's formulae, and the theo- 

 retical intensity of the light reflected at 45° determined. 



Speculum Metal. Steel. 



Observed 58*03 .... 44 '42 



Calculated 58*56 .... 48 '98 



The differences between the observed and calculated values are 

 nearly the same as when the measurements were made in air. 



Note by the Communicator. Received February 13, 1883. 



The differences between the results of theory and observation as to 

 the intensity of reflected light are in several of the above experiments 

 so considerable, that we are led to ask whether there may not be some- 

 thing in the experiments to which it may be referred. 



A slight inaccuracy in the calculated numbers is produced by the 

 neglect of the polarisation due to oblique reflection from the paper 

 •employed. With glazed paper this might be considerable ; but naturally 

 a paper would be selected with as dull a surface as possible, and the 

 author assures me that the polarisation was only just perceptible. It 

 could be easily allowed for by measuring the amount of polarisation 

 produced by the oblique reflection. When light of given intensity 

 polarised first in and then perpendicularly to the plane of incidence is 

 incident obliquely on the paper, let the intensities of the reflected 



