1890.] Photometric Observations of the Sun and Sky. 



Table C. 



(Sun's Altitude = 42 28'.) 



weather) in the plane of symmetry. The barrels of the mitrailleuse 

 were fixed 10 apart, the altitude of the sun being 42 28'. 



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. be the 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 Q. 



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

 that the value, ia cosec 0, gives the intensity very accurately, for any 

 point, in any other great circle, whose distance from the sun is 

 measured on that circle. 



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

 is a minimum at all points in a great circle, the plane of which is at 

 right angles to the line joining its centre to the sun. 



[This plane I call the plane of minimum intensity (ta).] 



As the whole of the mathematical developments of this paper are 

 founded upon the law that at any point of the sky whose distance is 

 O 3 from the sun 



the intensity = ia. cosec 0, 



