1890.] Photometric Observations of the Sun and Sky. 





Total intensity of the gore on the paper at 



which is the formula I have used in numerical computations. 



It is the numerical value in the column " Sky alone " in Table B, 

 which is thus brought into direct verification with i a observed by the 

 mitrailleuse. 



Arts. 25 29 show that the integral (K) taken for the whole visible 

 hemisphere is 



2ia (ir sin a + 2 COS a) (Q). 



This is the whole chemical action of the hemisphere resolved on the 

 horizontal plane, which was one of the quantities observed by Sir H. 

 Roscoe. 



30. Deals with any suspicion that may arise that the law of 

 cosecants may have been assumed, the fact being that the law was 

 arrived at, by experiment simply, more than twenty-two years 

 ago, &c. 



31. Applies the equation (Y) to determine i a for the altitudes given 

 by Sir H. Roscoe in his table showing the total chemical action of 

 diffuse daylight (i.e., of the whole sky, the sun being stopped off) 

 on horizontally exposed paper (' Phil. Trans.,' 1870, p. 314). 

 These values are tabulated with corresponding values of ia calculated 

 by formula in (24) from the Dacca Table B, forming together 

 Table E. 



32. As a first approximation from Table E, it would appear that 

 Sir H. Boscoe's unit of chemical action is if of the Dacca candle unit- 



