PHENOMENA OF GROUPS OF SOLAR SPOTS. 189 



Since I first satisfied myself as to the reality of the 

 binary character of groups, I have found that in the case 

 of spots apparently single and isolated a careful examina- 

 tion with a larger aperture and higher power will often 

 reveal a minute spot at a distance on the following side, 

 which afterwards frequently proves to be the nucleus of a 

 following sub-group ; and though I formerly regarded 

 many spots as single and isolated, I now believe that, in 

 fact, such spots are of extremely rare occurrence. 



The two centres of force or activity in binary groups are 

 sometimes very widely separated, instances not uncom- 

 monly occurring in which the distance between them 

 exceeds ninety thousand miles. 



With regard to groups which exhibit anomalous appear- 

 ances and curious and complicated changes, I have some- 

 times observed that they arise from the interference of two 

 smaller groups which have broken out near each other, 

 and have gradually extended themselves until they formed 

 one compound group. I need not remark upon the im- 

 portance of a careful observation of the phenomena of 

 these compound groups. 



I have alluded to the observations of Mr. Dawes and 

 Professor Secchi on the rotation of spots, and I may add 

 that in all the cases of rotation which have come under 

 my own notice the rotating spot has been the principal 

 member of the preceding sub-group ; but I must remark 

 that the observations which I have hitherto made are very 

 far from being conclusive as to whether this rotation is 

 real or only apparent. 



It will not escape remark with reference to the binary 

 character of groups, that there is considerable difficulty in 

 conceiving the mode of operation by which the forces that 

 produce the spots in a group should first develope them- 

 selves, almost simultaneously, at two points so widely 

 distant from each other; and I must admit that in the 



