316 Lord Rayleigh on the Intensity of Light reflected from 



to be embarrassed by uncertainty as to the effective wave- 

 length of the light in operation. Moreover, as these plates 

 are scarcely sensitive to yellow and green light, the effective 

 wave-length is liable to considerable variation with the current 

 used to ignite the lamp. Photographs were indeed taken of 

 the spectrum of the lamp as actually employed, but the un- 

 symmetrical character of the falling off at the two ends made 

 it difficult to fit upon the centre of activity. Recourse was 

 then had to Edwards' " isochromatic " plates. The spectrum 

 of the lamp, as photographed upon these plates after passing 

 through a pale yellow glass, was very well defined, lying with 

 almost perfect symmetry between the sodium and the thallium 

 lines. It was, therefore, determined to use these plates and 

 the same yellow glass in the actual experiments, so that 



A = |(5892 + 5319) = 5620 



could be taken as the representative wave-length. 



The only disadvantage arising from this change was in the 

 necessary prolongation of the exposure, which became some- 

 what tedious. Although no dense image is required or indeed 

 desirable, the exposure should be such that the development 

 does not need to be forced. Two photographs, with different 

 times of screening, were usually taken upon the same plate, 

 the object being to obtain a reversal of relative intensity, so 

 that in one image the semicircle representing the direct light 

 should be more intense and in the other image the semicircle 

 representing the reflected light. The best way of examining 

 the pictures depended somewhat upon circumstances. When 

 the exposure and development had been suitable, the most 

 effective view for the detection of a feeble difference was 

 obtained by placing the dry picture, film downwards, upon a 

 piece of opal glass. The light returned to the eye had then 

 for the most part traversed the film twice, with the effect of 

 doubling any feeble difference which would occur on simple 

 transmission. Under favourable circumstances it was possible 

 to detect a reversal between the two images when the difference 

 amounted to oj per cent, A few such experiments might 

 therefore be expected to give the required result accurate to 

 less than one per cent. 



With the Edwards'' plates an exposure of 12 minutes was 

 found to be necessary. This was divided into two parts of 

 6 minutes each, with an interval of one minute during which 

 the mirrors were reversed. About the middle of each period 

 of 6 minutes the direct light was screened off for a time which 

 varied from picture to picture. For example, on June 6, the 

 time of screening for one picture was 71 seconds, and for the 



