5 o 4 THE POPULAR SCIENCE MONTHLY. 



the publication quaintly termed " Philosophical Transactions " of the 

 London Royal Society in 1796. 



Sir "W. Herschel here mentions the number of variable stars, con- 

 stantly increasing under new discoveries, very naturally predicts that 

 closer observation will be likely to show variability in objects pre- 

 viously unsuspected, and recommends that careful comparisons be 

 made from time to time between neighboring stars all over the heav- 

 ens, so that any change occurring may be at once detected. The 

 original comparisons accompanying this paper have been of but little 

 use, however ; they are interesting chiefly as having been the first at- 

 tempt to introduce scientific methods into this unexplored territory of 

 the astronomical realm. They were made without the aid of any in- 

 strument, and consisted of such indefinite statements as " Star No. 

 7 about equal to No. 4, and just perceptibly fainter, or decidedly 

 brighter, than No. 12." The difference of brightness which Herschel 

 considered as "just perceptible" seems to have been from one fifth to 

 one fourth of a magnitude. 



That his least appreciable difference should have some constant re- 

 lation to the traditional " magnitude " was to have been expected, bear- 

 ing in mind what this oldest and most universal scale of reference was 

 intended to express. The fixed stars were assigned to classes of bright- 

 ness, we learn, before the Christian era ; and the very term " mag- 

 nitudes," used from the first to designate these classes, shows the state 

 of knowledge under which the study had its origin, for, as we now 

 know, the apparently greater size of the brighter stars is due only to 

 imperfections of the eye. All visible stars all that existed, that is, 

 for the early astronomers of the Mediterranean were included in six 

 magnitudes, the first containing the dozen or score of brightest stars 

 in the heavens, the second perhaps twice as many ranking next to 

 these, and so on out in gradually increasing circles. The work of the 

 ancients has in this case been well preserved, no modern innovator 

 having been found bold enough to disturb this time-honored system 

 of reckoning. Still, as in the days of the " Father of Astronomy," the 

 two chief stars of Orion serve as examples of the first magnitude, while 

 his Belt and the Dipper in the northern sky furnish types of the second 

 order. But, while astronomy was yet in its infancy, observers had 

 noticed that the stars were not assorted into well-defined orders, in 

 which all the individuals were equally bright ; and so, in assigning to 

 a star its magnitude, they would often add that it was " smaller " or 

 " larger " than the mean of that magnitude. They thus practically 

 trebled the number of their classes. The same division into thirds of 

 a magnitude is still employed by those who judge of brightness by eye- 

 estimates, though some are content with dividing into halves, and some 

 undertake to be exact to tenths. Now, even though no scientific pre- 

 cision was attained, or even thought of, in this original apportionment 

 of visible stars among the six magnitudes, to which all later estimates 



