PLEASURES OF THE TELESCOPE. 793 



of the lines in its spectrum. By means of data thus obtained the 

 mass, size, and distance apart of Algol and its singular comrade 

 have been inferred. The diameter of Algol is believed to be 

 about 1,125,000 miles, that of the dark body about 840,000 miles, 

 and the mean distance from center to center 3,230,000 miles. The 

 density of both the light and the dark star is slight compared 

 with that of the sun, so that their combined mass is only two 

 thirds as great as the sun's. 



Mention has been made of a slight irregularity in Algol's 

 period of variation. Basing his calculations upon this inequality 

 Dr. Chandler has put forward the hypothesis that there is another 

 invisible body connected with Algol, and situated at a distance 

 from it of about 1,800,000,000 miles, and that around this body, 

 which is far more mssive than the others, Algol and its com- 

 panions revolve in a period of one hundred and thirty years ! 

 Dr. Chandler has earned the right to have his hypotheses re- 

 garded with respect, even when they are as extraordinary as that 

 which has just been described. It needs no indulgence of the 

 imagination to lend interest to Algol ; the simple facts are suffi- 

 cient. How did that bright star fall in with its black neighbors ? 

 Or were they created together ? 



Passing to the region covered by map No. 25, our eyes are 

 caught by the curious figure, formed by the five brightest stars 

 of the constellation Cassiopeia, somewhat resembling the letter 

 W. Like Perseus, this is a rich constellation, both in star clusters 

 and double stars. Among the latter we select as our first ex- 

 ample o-, in which we find a combination of color that is at once 

 very unusual and very striking green and blue. The magni- 

 tudes are five and seven, distance 3", p. 324. Another beautiful 

 colored double is rj, whose magnitudes are four and seven and a 

 half, distance 5", p. 200, colors white and purple. This is one 

 of the comparatively small number of stars the measure of whose 

 distance has been attempted, and a keen sense of the uncertainty 

 of such measures is conveyed by the fact that authorities of ap- 

 parently equal weight place t\ Cassiopeise at such discordant dis- 

 tances as 124,000,000,000,000 miles, 70,000,000,000,000 miles, and 

 42,000,000,000,000 miles. It will be observed that the difference 

 between the greatest and the least of these estimates is about 

 double the entire distance given by the latter. The same thing 

 practically is true of the various attempts to ascertain the dis- 

 tance of the other stars which have a perceptible parallax, even 

 those which are evidently the nearest. In some cases the later 

 measures increase the distance, in other cases they diminish it ; 

 in no case is there anything like a complete accord. Yet of 

 course we are not to infer that it is hopeless to learn anything 

 about the distances of the stars. With all their uncertainties and 



