428 PROCEEDINGS OF THE AMERICAN ACADEMY 



the equation y=imx^, the light being proportional to some power of 

 the sun's angular distance. 



The most noticeable deviation from a straight lyie is in the curve 

 for series I. where it approaches the axis of y ; here the intensities are 

 less than required by a straiglit line, which is explained by the fact 

 that this series was made late in the afternoon of a winter day when 

 the light of the sun itself was rapidly diminishing, and, as before stated, 

 the observations nearest the sun were made last. 



Neglecting, for the reason just given, the first three observations of 

 series I., we obtain tlie following as the most probable values of n in 

 each case: series I. — 1.4; II. — 1.4; III. — 1.32; IV. — 1.21. Com- 

 puting now the numerical value of m for each series, and substituting 

 in the equation y = ???x", we have for series : \. y=. 359x~^* ; II. y = 

 33.1X-1* ; III. y = IS.Ta--^-^^ ; IV. y = 12.6x-i-2i, 



Columns 1, 2, 3, and 4 give the deviations from the formulae of 

 the observations of series I., II., III., and IV. respectively ; the devia- 

 tions being expressed in pei'centages of the intensities. The last 

 column gives the mean of the deviations ; neglecting, as before, the first 

 three observations of series I. Although some of the deviations are 

 quite large, yet the sums of the positive and negative deviations are 

 approximately equal ; and it will be observed that they frequently 

 change their sign, which shows a close agreement with theory. It is 

 probable that the larger deviations are attributable in part, at least, to 

 sudden changes in the reflecting power of the sky, such as would be 

 produced by air currents or by the precipitation or dissipation of atmos- 

 pheric moisture. 



That these deviations are greater than those due to errors of obser- 

 vation, is clearly shown by the experiments in the preceding article. 



v. — LIGHT ABSOEBED BY THE ATMOSPHEEE OF THE SUN. 



By E. C. Pickering and D. P. Strange. 



« 



The following series of experiments were made for the purpose of 

 determining the relative amount of light received from portions of the 

 sun's surface at varying distances from the centre of its disk. For this 

 purpose, the sun's rays were reflected into a darkened room by means 

 of the black glass mirror of a porte-lumiere, and an image of the sun, 

 40 cms. in diameter, was, by means of a small telescope, thrown upon 

 a screen placed at a distance of 230 cms. from the aperture. In the 



