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 one direction, 

 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 a*nd 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). 1 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. 



1 In discussing the periodicities of sunspots (1906 2 , 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. 



