Photometric Observation* of the Sun and Sky. 265 



The readings of the chemical action are taken in terms of the unit 

 of candle power, and were compared also with a graded Sun-strip, 

 made at the same time from the same photographic paper by the 

 water-motion actinometer, fig. 1. 



The observations given by the barrels at 170 and 10 are too low, 

 doubtless owing to haze so near the horizon. No observation could be 

 made with the barrel at 40, because the Sun could not be kept out of 

 it. The observation made by the barrel at 20" is (apart from com- 

 parison with computed value) evidently erroneously large. I give 

 the table as an early observation that shows well that there is a point 

 of minimum sky intensity at 90 from the Sun. It also appears that 

 if ia represent this intensity for the altitude of the Sun (= 0'12), 

 then the intensity of the sky at a point 6 from the Sun is given 

 (roughly only according to this table) by the formula 



ia cosec 6. 



This observation was made in the plane of symmetry : it turns out 

 that the value, i a cosec 6, gives the intensity very accurately, in 

 whatever plane 9 be measured from the Sun. 



I would note once more that my observations are largely compara- 

 tive, and the results obtained are independent of the unit : it is not 

 accessary to reduce the readings in this table to the one- second unit. 



20. For any altitude of the Sun (), the chemical action of the sky 

 a minimum at all points of a great circle 90 from the Sun, the 



jlane of which is the plane of minimum intensity (ia). And at this 

 loment, the chemical action of the sky at any point distant from 

 Sun is given with great accuracy by the formula 



ia cosec 6. 



As the whole of the mathematical developments of this paper are 

 junded on this law, I have been careful not only to verify it by 

 inmerous observations both at Dacca and in Somersetshire, but also 

 vary the form of the observations in every way I could devise. 



21. Thus, the mitrailleuse has been placed in the plane of minimum 

 itensity : in this case, all the barrels give accurately the same read- 

 ier, except that those bai'rels 10 from the horizon read rather lower, 



I anticipated they would ; there must nearly always be some haze 

 jar the horizon. 



Next, the mitrailleuse was placed at various angles with the plane 

 symmetry, by turning it round the line joining one of its tubes 

 nth the Sun. The observed chemical actions agree well with 

 cosec 6. Next, by means of stops, I made the aperture of each 

 irrel of a mitrailleuse to bo c sin 0, where 9 is the distance of the 

 sis of the barrel from the Sun. This mitrailleuse being exposed, 



T 2 



