88 CLIMATIC CYCLES AND TREE-GROWTH. 
But the photographic summation may be done at any slant instead 
of only in the vertical, and therefore the sensitive plate may be made 
to summate these curves through a long range of periods. In order to 
get a long range of periods, the diagram was mounted on an axis with 
clock-work and slowly rotated in front of a camera with a cylindrical 
lens for objective, a horizontal slit in the focal plane, and a sensitive 
plate passing slowly downward across the slit by clock mechanism. 
In this way a full range of possible periods come under the summing 
process, and when a real period is vertical the crests of the curves form 
vertical lines which come down as a series of dots or beads in the slit. 
When no period is in the vertical the light coming through the slit 
is uniform. Of course, there is a practical limit to the different angles 
at which the diagram may be viewed. An angle too far in onedirection, 
making the tested period very small, would require a great number of 
duplications of the curve, while too great an angle the other way, mak- 
ing the tested period very large, catches the curve in the nonsymmetri- 
cal form and introduces errors. In the periodograms actually made of 
the sunspot curve the minimum period tested was 7 years and the 
maximum 24. One notes especially that this is a continuous process 
and that all periods from the minimum to the maximum are tested. 
Application to length of sunspot period.—The interest in the sun- 
spot period makes a special consideration of plate 9, c, worth while. 
This figure is a photograph of plate 9, B, taken out of focus for the pur- 
pose of calling attention to certain general features. In B the eye 
naturally turns to the sharp outlines and notes its minute details. In 
c the crests of B are changed into large blotches connecting somewhat 
with their nearest neighbors and varying in intensity. The alinement 
which they form in a nearly vertical direction is a graphic representa- 
tion of the period. If the line were exactly vertical the period would be 
10 years. The slant to the right shows more than 11. If the line were 
straight the period would be constant. It is evident that there are 
several irregularities in it. Having a number of exactly similar lines 
side by side, the irregularities are repeated in each and thus strike the 
consciousness with the effect of repeated blows. These irregularities 
are the discontinuities referred to by Turner in connection with his 
hypothesis. It is evident at a glance that the sunspot sequence 
divides itself into three parts, namely, a 9.3-year period, 1750-1790; 
then an interval of readjustment, 1800-1830, with a 13-year period; 
and lastly an 11.4-year period lasting to the present time (values 
approximate).'! But the latter is not perfectly constant, for after 1870 
there is a change in intensity. The breaks thus shown and Turner’s 
dates of discontinuity are compared in table 6. 
1In discussing the periodicities of sunspots (1906*, pp. 75-78) Schuster divided his 150 years, 
from 1750 to 1900, into two nearly equal parts. He found in the first part two periods of 9.25 
and 13.75 years acting successively, and in the second part, a period of 11.1 years. 
