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POPULAR SCIENCE MONTHLY. 



tween the periodic stars and the irregular variables to be hereafter 

 described. 



Keturning to the tipper end of the scale, the contrast between the 

 great number of stars less than fifty days, and the small number be- 

 tween fifty and one hundred, seems to show that we have here a sharp 

 line of distinction between stars of long and those of short period. 

 But, when we examine the matter in detail, the statistics of the periods 

 do not enable us to draw any such line. About eight periods are less 

 than one day, and the number of this class known to us is continually 

 increasing. About forty are between one and ten days, and from this 

 point upwards they are scattered with a fair approach to equality up 

 to a period of one hundred days. There is, however, a possible dis- 

 tinction, which we shall develop presently. 



The law of change in a variable star is represented to the eye by a 

 curve in the following way. We draw a straight horizontal line A X to 



ti 



abed 



Fig. 1. The Law of Change in a Variable Star. 



represent the time. A series of equidistant points, a, I, c, d, etc., on 

 this will represent moments of time. One of the spaces, a, b, c, etc., 

 may represent an hour, a day, or a month, according to the rapidity of 

 change. We take a to represent the initial moment, and erect an ordi- 

 nate aa',oi such length as to represent the brightness of the star on some 

 convenient scale at this moment. At the second moment, &, which may 

 be an hour or a day later, we erect another ordinate W, representing 

 the brightness at this moment. We continue this process as long as 

 may be required. Then we draw a curve, represented by the dotted 

 line, through the ends of all the ordinates. In the case of a periodic 

 star it is only necessary to draw the curve through a single period, since 

 its continuation will be a repetition of its form for any one period. 



We readily see that if a star does not vary, all the ordinates will be 

 of equal length, and the curve will be a horizontal straight line. More- 

 over, the curve will take this form through any portion of time during 

 which the light of the star is constant. 



There are three of the periodic stars plainly visible to the naked 

 eye at maximum, of which the variations are so wide that they may 



