Photometric Observations of the Sun and Sky. 



2(39 



"able B, which is thus brought into direct verification with ?' a , 

 observed by the mitrailleuse. 



An example of the actual calculation of ia. is added in Appendix B, 

 not for publication. 



25. The values for I u I G for different altitudes of the Sun in 

 Table B are much the most trustworthy observations, and are the 



leans obtained from a very large number of observations. I have, 

 therefore, by the formula obtained in the last paragraph (24), in- 

 versely calculated the value of ia. for every 5 within the limits 5 to 

 40, and placed them in Table D. 



Table D. 



i a calculated from Table B 

 Sun's altitude. (column headed " Sky alone"). 



5 -00329 



10 -00681 



15 -00928 



20 -01073 



25 -01144 



30 -01188 



35 -01205 



40 -01218 



45 -01213 



50 '01209 



55 -01204 



60 -01200 



65 -01195 



26. Theorem. On the resolution of the chemical action of the sky 

 in a direction perpendicular to any plane. 



The figure (Dia. 2) is supposed an orthographic projection of the 



/isible hemisphere on the plane of the horizon ; S being the Sun, Z 



the zenith, HSZM the projection of the plane of symmetry, M'MI that 



)f the plane of minimum intensity, and M'SI that of the plane through 



at right angles to each of the other planes (which I call the plane 



)f the Sun's altitude). These three planes, when produced, divide 



le sphere into eight quadrantal surfaces, of which SMI is one. In 



le quadrantal triangle SMI, S, M, I are the poles of the opposite 



sides. 



Let the polar coordinates of P (an element of the surface) be 

 'SZ = and SP = 0. Then, as before, the element will have an area 



dO. sin = ia. df}>. dO. 

 Let the planes OSM, OSI, and OIM (O being the centre of the 

 ^misphere) be taken as coordinate planes ; OS, OM, 01, the three axes 



