40 



Sir John Conroy. 



[May 15, 



its thickness was therefore 0'0001737 millim., or as nearly as possible 

 double that of the single film. 



The thick film was not absolutely opaque, as the disk of the sun on 

 a clear day could just be seen through it, but it transmitted much less 

 light than the film previously used. 



Tables IV and V give the results of two series of observations 

 made with it, and also the theoretical amount of light which should 

 have been reflected, calculated from the values of the principal 

 incidence and principal azimuth given in Table VI. 



Table V. Double Silver Film, with Light polarised perpendicularly 

 to the Plane of Incidence. 



Table VI. 

 Principal incidence. 



75 50' 



75 45 



Mean . . .75 47 



Principal azimuth. 

 43 52' 

 44 07 



44 



The values of the principal incidence and azimuth are slightly 

 higher than those obtained with the thinner film, and therefore the 

 percentage amount of light which, according to theory, should be 

 reflected by the silver, is also higher. 



The tables show that both for light polarised in and perpendi- 

 cularly to the plane of incidence the observed intensity exceeds the 

 calculated intensity, in the former case by about 1, and in the latter 

 by about 2 per cent., except at incidences of 30 with light polarised 

 in the plane, and 70 and 75 for light polarised perpendicularly to 

 the plane, for which angles the observed and calculated intensities 

 agree closely. 



Curves were drawn to show the calculated and observed intensities, 



