86 



CLIMATIC CYCLES AND TREE-GROWTH. 



scale could easily be cast in all sizes upon a plotted curve. But all 

 these methods of equal spacing on a plotted curve leave far too much 

 to the individual judgment of the investigator. 



THE OPTICAL PERIODOGRAPH. 



A method of periodic analysis well adapted to the work in hand has 

 been developed by the writer as the need for it became more and more 

 evident. Along with the feeling of need for rapid analysis was the 

 increasing recognition of the desirability of some process which would 

 place mere individual judgment and personal equation as far in the 

 background as possible. 



Schuster's periodogram. In 1898, Schuster suggested the use of the 

 word " periodogram" as analogous to the word spectrogram; that is, a 

 periodogram is a curve or a photograph which indicates the intensity 

 of time periods just as the spectrogram indicates intensity of space- 

 periods or wave-lengths. The spectrogram commonly gives its inten- 

 sities by varying photographic density along a band of progressive 

 wave-lengths. For the periodogram Schuster made simply a plotted 

 curve, of which the abscissae represented progressive time-periods 

 and the ordinates represented intensities. He made a mathematical 

 analysis of the sunspot numbers and constructed a periodogram which 

 is reproduced in figure 30. It shows periods at its crest at 4.38, 4.80, 

 8.36, 11.125, and 13.50 years. 



woo 



3000 



zooo 



1000 



FIG. 30. Schuster's periodogram of the sunspot numbers. 



The optical periodogram. It is, of course, not necessary that the 

 periodogram should take the form of a plotted curve with intensities 

 represented by ordinates, nor yet need it be exactly like a spectrogram 

 showing intensities by density. The first periodogram produced by 

 the writer is shown in plate 9, A. It is an analysis of the sunspot num- 



