Photometric Observations of the Sun and Ski/. 



259 



was taken on 21st December, 1868, on the roof of my house at 

 Dacca, the sky being perfectly clear. The paper was exposed at right 

 angles to the Sun, thus giving the effect of the Sun, together with the 

 total effect (resolved on the plane at right angles to the Sun) of that 

 portion of the visible sky within 90 of the Sun. 



Table A. 



the two numbers in the third and fourth columns. 



hus, taking the last but one observation, 

 6-7 xO : 56 = 3752, 



wm = ' 266 - 



The constant C of shading used as the standard of comparison 

 was the tint produced in the same paper by the candle burning 47 

 minutes at 1 inch distance. Hence, the unit I here employed was 

 47 x 60 times i the unit in paragraph 4 above. 



.11 order to get a deeper shade of darkening in the first two obser- 

 ions, when the Sun was low, a smaller stopcock was used than in 

 succeeding observations. 



In each of these observations, the actual velocity of the slide was 

 rved by an assistant with a watch. As explained in paragraph 7, 

 is constant can be obtained more easily and exactly by a gauge, 

 asted on the plane beside the slit, graduated for the stopcock 



