1863.] 



649 



the sun's disc is nearly double that from its borders, and that the 

 equatorial regions are somewhat hotter than the polar, whilst 

 observers have long noticed a great difference in luminosity between 

 the centre and edge of the disc. 



For the purpose of obtaining a measurement of the relative 

 chemical brightness of various portions of the solar disc, the image 

 of the sun, of about 4 inches in diameter, obtained by a 3|-inch 

 refractor*, was allowed to fall into a camera placed on the 

 instrument, upon a sheet of standard photographic paper prepared 

 according to the method described in the above-mentioned research. 

 The peculiar property of this standard paper is that it can always 

 be prepared of one and the same degree of sensitiveness, and is per- 

 fectly homogeneous. The exposure lasted for from 30 to 120 

 seconds, the sun's motion being carefully followed by a tangent-screw* 

 After exposure, the shade of tint at several points on the picture was 

 determined by comparison with a graduated photographic strip in- 

 solated in the pendulum-photometer, and the chemical intensities 

 corresponding to these shades obtained by reference to the Table 

 given in the memoir above cited. The following numbers give the 

 chemical brightness, thus obtained, at various points on the sun's 

 disc on May 9th, 1863. From these numbers it is seen that the 

 intensity of the chemically active rays at the centre is from three to 

 five times as great as that at the edge of the disc, the chemical rays 

 thus showing a wider variation than the calorific rays exhibited as 

 determined by Secchi. This is doubtless owing to the relatively 

 greater absorption eifected by the solar atmosphere on the more 

 refrangible chemical rays. 



Chemical Brightness of Sun's Disc on May 9, 1863. 



Hence it is likewise seen that on May 9th the chemical bright- 

 ness of the south polar regions was considerably greater than that of 

 the north polar regions, whilst about the equator the brightness was 

 between that of the poles. 



* Kindly placed at my disposal by S. W. Williamson, Esq., of Manchester. 



