314 



PROFESSOR L. BECKER ON THE DISTRIBUTION OF 



subtended by BA at M. The position angle, counted from tbe north pole of the sun, 

 of the second contact is 104 degrees. CMS is designated by a. 



A measured distance m, at position-angle P, is reduced to distance h from the sun's 

 limb by the following formula, small quantities being neglected, 



tan a 



./I- * 

 V 103 



tan 7 8 -3, /t = 



125 1- 



cos 7 8 -3 

 cos a 



cos (P 



-104-78)1. 



The maximum of h m is 0'025 inch. The correction of the position-angles is 

 inappreciable for our purpose. 



(c) Curves of Equal Intensity of the Corona. I define the mean distance of an 

 equal-intensity curve of the corona as the mean of the distances of twenty-four points 

 of the curve, 15 degrees apart. Equal-blackness curves coincide with equal-intensity 

 curves on Photographs VIII. and IX., and also on Photograph I. The measured 

 distances were first corrected for corrections (a) and (6), and then each twenty-four 



Fig. 4. 



Fig. 5. ' 



distances belonging to an equal-intensity (or blackness) curve were combined to a 

 mean ; the differences, 8A, (mean minus reduced distance) define the equal-intensity 

 (or blackness) curve with reference to the circular mean curve. Finally curves were 

 interpolated from the observed 95 curves at regular intervals of the mean distance. 

 An extract of the results is contained in Table I. (p. 337), and graphs of some of the 

 curves are shown in fig. 5. 



(d) Reduction of the Distances of Portions of an Equal-blackness Curve to the 

 Mean Distance of that Curve. Though equal-blackness curves were measured on all 

 pictures at all position angles, only portions of these curves can be used together, 

 because the equal-blackness curves do not everywhere coincide with the equal- 

 intensity curves of the corona. In next section it will be shown (l) that in the case 

 of Photographs V., VI, and VII., owing to luminosity of the sky, the left and right 



