The Solar Activity 1833-1900. 



285 



of focus by moving the screen 3 or 4 cm, nearer to the lens, there 

 appear two patches, one red the other green, which overlap one 

 another, the part common to both being yellow. In the pulsative 

 image the red and green become respectively blue-green and purple, 

 while the overlapping portion is almost colourless. Possibly both the 

 pulsative colours are less blue than they should be, with the result 

 that their combination produces white or grey. 



The difficulty of forming a satisfactory pulsative image from blue 

 and violet is no doubt to be accounted for by the superior persistence 

 of those colours. With stronger luminosity than can be obtained by 

 the method of projection or by the use of pigments this difficulty is 

 diminished, for then the greater part of the luminous impression 

 vanishes more quickly. 



Though the work of which an account is given in the present paper 

 has occupied a large amount of time, it is obvious that the subject is 

 far from being exhausted. Several doubtful points remain to be 

 cleared up and apparent discrepancies reconciled, while of a number 

 of remarkable phenomena which presented themselves no mention at 

 all has been made. With more refined apparatus than that at present 

 at my disposal, similar methods of experiment might be expected to 

 yield important contributions to the theory of colour- vision. 



" The Solar Activity 1833-1900." By William J. S. Lockyer, 

 M.A., Ph.D., F.B.A.S., Assistant Director, Solar Physics 

 Observatory, Kensington. Communicated by Sir Norman 

 Lockyer, K.C.B., F.E.S. Eeceived April 29,— Eead May 23, 

 1901. 



Introduction. 



A close examination of the curves representing the varying amount 

 of spotted area on the Sun's surface, shows that no two successive 

 cycles are alike either in form or area. The individuality of the cycles 

 seems, on further inspection, to be repeated after a certain period of 

 time, and this peculiarity, coupled with a like variation in the curves 

 representing the variations of the magnetic elements, and with suspected 

 cycles of change in various terrestrial phenomena, suggested a new 

 investigation of the whole subject. 



The object of this communication is to place before the Eoyal 

 Society the first results which an examination of the various records 

 has furnished. 



Dr. Eudolf Wolf,* of Zurich, from a study of the sunspot observa- 

 tions made up to the end of 1875, drew attention to the facts, to use 



**' Mem. R. Astron. Soc.,' vol. 43, p. 200. 



x 2 



