1881.] 



Some Experiments on Metallic Reflexion. 



493 



incidences and azimuths being somewhat higher, especially with blue 

 light, in the case of the gold leaf. 



It therefore appears probable that of the two sets of values for the 

 gold plate, those obtained after the plate had been polished with putty 

 powder are nearest the truth. 



A large number of determinations of the principal incidences and 

 principal azimuths were made with a silver plate in air, water, and 

 carbon tetrachloride, with both the quarter undulation plates. 



The silver plate had been polished with rouge, and during the course 

 of these experiments, which were carried on at intervals during some 

 months, rubbed with a little rouge from time to time, and although 

 afterwards well rubbed on a clean chamois leather, in certain lights it 

 appeared to have a reddish tinge, although the surface was most 

 brilliant. 



In some few of the earlier experiments sixteen observations were 

 made in each position of the quarter plate, four with the polariser in 

 each quadrant ; but most of the numbers are the means of eight 

 observations, two for each position of the polariser. 



The following table contains the measurements made with the 

 quarter plate VI for red and yellow light, and with quarter plate I 

 for blue light : — 



Silver in air. 

 I. 



Silver in air. 

 II. 



Silver in 

 water. 



< Plate at B. 7 



r Plate at A. 



r Plate at A. 

 i Plate at B. 



Silver in r Plate at A. 



carbon < Plate at B. 



tetrachloride. L Mean value 7 



Bed light. 



Yellow light. 



Blue light. 



P.I. 



P.A. 



P.I. 



P.A. 



P.I. 



P.A. 





o / 











78 13 



32 50 



76 06 



35 40 



70 06 



38 43 



75 04 



31 58 



75 40 



35 44 



73 16 



39 26 



76 38 



32 25 



75 53 



35 42 



71 41 



39 04 



77 51 



33 01 



75 42 



35 10 



72 16 



38 45 



74 



32 47 



75 21 



34 59 



72 24 



38 46 



75 55 



32 54 



75 31 



35 04 



72 20 



38 45 



75 13 



33 42 



73 11 



36 47 



67 09 



40 06 



71 16 



33 12 



72 20 



35 27 



69 08 



40 07 



73 14 



33 27 



72 45 



36 07 



68 08 



40 06 



75 04 



32 52 



70 27 



35 41 



64 32 



39 36 



71 01 



32 34 



70 50 



35 22 



67 32 



39 10 



73 02 



32 43 



70 38 



35 31 



66 02 



39 23 



The values of the principal incidences and principal azimuths were 

 also determined by a somewhat different method. The principal 

 section of the polarising nicol was placed successively in the plane of 

 incidence, and at right angles to it, and the neutral axes of the quarter 

 undulation plate (plate VI being used) at an angle of 45° with this 

 plane. 



The angle of incidence, and the azimuth of the analysing nicol by 



