OP ARTS AND SCIENCES. 283 



of absorption is in general shown in their spectra bcvonil the appear- 

 ance of lines snch as are seen in our Sun. For the stars of tlie second 

 class, however, this view seems more reasonable, since many of them 

 exhibit specti'a which are strongly banded. Moreover, tlie great varia- 

 tion in light is thus explained. An excellent test of this hypothesis is 

 atlbrded by the variation in light of the different portions of the spectra. 

 For light of any given wave-length the logarithm of the transmitted 

 lay will always vary proportionally to the thickness and density of the 

 absorbing medium, the amount of absorbent effect for any given thick- 

 ness varying with the wave-length. Accordingly, a study of the varia- 

 tion of each ray should show the came law. They would give very 

 different coeilicients of absorptions, those of the dark bands being 

 large, and those of the bright zones being small. The great variation 

 in light will render this test a severe one with even a moderate degree 

 of accuracy in the observations. For the lack of any data, this method 

 of study is for the present unavoidably postponed. 



The principal conclusions of the above paper may be summarized 

 as follows : — 



Thirty-one variable stars are known whose period is less than 72 

 days. Of those six belong to the fifth class, or that of /3 Persei, in 

 which the variation is probably due to the interposition of an opaque 

 eclipsing satellite. Of the remainder, seven may be excluded, since 

 they are red, and may belong to the second class, or that of o Ceti. 

 Nineteen remain, whose periods vary from less than a day to 54 days, 

 and which may be placed in the fourth class. All lie within 16° of a 

 great circle whose pole is in R. A. 13^, Dec. -|- 20°. The distances 

 of eleven are from 0° to 5°, of five at distances 8° and 9°, one at 14°, 

 and one at 16°. The average distance is 5°.o, while if the stars were 

 distributed at random it should be 30°. 



If the stars of tlie Durchmusterung were uniformly distributed, their 

 average distance apart would be about 8'.7. The five stars of the fifth 

 class have Durchmusterung companions at au average distance of lO'.G. 

 In the fourth class, excluding the red stars, six are in the Durch- 

 musterung, and have companions at an average distance of 2'.5, four 

 being less than 2'.0 distance, one at 3'.2, and one at 6M. In all six 

 cases the direction of the companions is within less than 34° of the 

 plane near which the variables lie, or at an average distance of 18°, 

 while, if distributed by chance, this angle should be 45°. Hence 

 a method of discovering variable stars of this class is offered by look- 

 ing in a certain part of the sky for those having near companions in 

 a given direction. 



