September 15, 1911] 



%GIENCE 



331 



of analysis corresponds to the determina- 

 tion of the complete spectrum of the source 

 of light, which may consist of bright lines 

 superimposed on a continuous spectrum. 

 And the length of the series of observations 

 corresponds simply to the resolving power 

 of the optical apparatus. The only point 

 in which the analogy breaks down is unfor- 

 tunately that of ease and simplicity. In 

 the optical analogy, an optical instrument 

 performs for us with completeness and 

 despatch the analysis, which in its counter- 

 part must be performed by ourselves with 

 much numerical labor. 



Let us consider how we should most con- 

 veniently proceed to the complete delinea- 

 tion of a spectrum. "We should ultimately 

 need an apparatus of the greatest possible 

 resolving power, but it might not be ad- 

 visable to begin with it: on the contrary, 

 a small instrument which enabled us to 

 glance through the whole spectrum might 

 save much time. Suppose, for instance, 

 that there was a bright line in the yellow; 

 our small instrument might suffice to show 

 us that it was due either to sodium or 

 helium, but no more : the decision between 

 these alternatives must be reserved for the 

 larger instrument. On the other hand, if 

 no line is seen in the yellow at all, we have 

 ruled out both possibilities at once, and so 

 economized labor. Hence it is natural to 

 use first an instrument of low resolving 

 power and afterwards one of higher. 



Now in the work for which this serves as 

 an analogy this procedure is actually im- 

 posed upon us by the march of events. It 

 has been pointed out that the resolving 

 power of the optical apparatus corresponds 

 exactly to the length of our series of ob- 

 servations. Hence our resolving power is 

 continually increasing. Quite naturally we 

 begin with a short series of observations, 

 which shows us our lines blurred and con- 

 fused: to define and resolve them we have 



but one resource — "wait and see"; wait 

 and accumulate more observations, to 

 lengthen the series. But the lengthening 

 must be in geometrical progression: we 

 must double our series to increase the re- 

 solving power in a definite ratio; and 

 double it again. We begin to get a glimpse 

 of the important part to be played by the 

 observer in the future, and of his increase 

 in numbers. 



Let us glance at a few illustrations of 

 the use of this method. Professor Schuster 

 has applied it, for instance, to the observa- 

 tions of sunspots. Now it may fairly be 

 said that the general law of sunspots was 

 thought to be known : the variation in a 

 cycle of about 11^ years has long been con- 

 sidered to represent the facts: it catches 

 the eye at once in a diagram, and though 

 there are also obvious anomalies, they had 

 not been deemed worthy of any particular 

 attention (with one exception presently to 

 be mentioned), until Professor Schuster 

 undertook his analysis. To his surprise, 

 when he calculated the periodogram of 

 sunspots, he found two entirely new facts: 

 (1) that there were other distinct periodici- 

 ties, notably of about four, eight and four- 

 teen years; (2) that the eleven-year cycle 

 had not been continuously in action, but 

 that during the eighteenth century it had 

 been much less marked than the eight-year 

 and fourteen-year cycles. 



A further most interesting fact seems to 

 emerge, viz. : that several of the periodici- 

 ties are harmonics of a major period of 

 some thirty-three years or more, and it 

 seems just possible that a connection may 

 ultimately be established with the Leonid 

 meteor-swarm, which revolves in this pe- 

 riod. But it would take us too far from 

 our main point to follow these most inter- 

 esting corollaries : the point well worthy of 

 our special attention is this, that we have 

 here an undoubted advance in knowledge 



