1893.] Sun Light, Cloud Light, and Candle Light. 3 



white light reflected from the first surface of the first prism was 

 again reflected from the surface of a thin prism on to the cube, a rod 

 placed in its path cast a shadow on that part illuminated by the sky 

 light, and by suitable adjustment the boundaries of the two shadows 

 were caused to exactly coincide. The colour was thus diluted with 

 white light, and rotating sectors, described in other papers, being 

 placed in the path of the white beam, enabled the dilution to be 

 regulated. 



Sky Light. On June 27, 1892, at 2.30 P.M., the sky was a good 

 blue, but not a dark blue, and perhaps rather milky. The slit was 

 moved into the part of the spectrum which appeared to be near the 

 dominant colour. The colour was diluted to approximately the required 

 amount. The slit was shifted and the dilution altered until the two 

 colours made a perfect match. It was found that on the standard 

 scale of the spectrum the dominant colour was represented by 28'6, 

 which is X 4800. The mean value of the sector aperture was 32, and 

 recollecting that the sectors are double sectors the comparison has to 

 be made with 180. The next operation was to compare the 

 luminosity of the whole beam of white light with that of the colour. 

 The sectors still remained in the white ; the sky light was cat off, 

 and the rod altered till the colour and the white were alongside each 

 other with the boundaries of the shadows touching. The luminosi- 

 ties of the two were compared, and it was found that the aperture of 

 the sector was 14. As it required 32 of white to make the dilution 

 of the colour, it follows that 32/14, or 2*286, parts of white were 

 required to dilute 1 part of the blue. This may be expressed thus 



Sky light = X 4800 + 2-3 W. 



On July 4, 1892, at mid-day, the same procedure was adopted, and 

 the dominant wave-length was again X 4800. In this case the amount 

 of added white was thus 



Sky light = X 4800 + 3'1 W; 



in other words, the sky was more milky. 



At 4 P.M. on the same day the sky to the east, and about 30 above 

 the horizon, was evidently slightly greener, and it was found that the 

 colour agreed with scale No. 29'6, or X 4834, and that it required 

 3 parts of white to be mixed with it. 



Skylight = X4834 + 3W. 



On other days, with the light of the portion of the sky near the 

 zone of maximum polarisation the dominant wave-length was found 

 to be between these two limits, and was never found bluer, and the 

 smallest admixture of white light was found to be 1*9. 



B 2 



