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 abscisse 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. 
4000 
Be | 
| 
s 
| 
1000 
piel 
A METS a. 
ore é 10 lz Me 16 18 20 22 
Yeors 
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- 
