242 TRANSACTIONS OF THE [mAY 17, 



Spoerer especially, in a statistical way, and by Vogel, Lohse, 

 Tacchini, and others, as to structure and detail. Spoerer has 

 brought out very clearly the connection between the number and 

 average latitude of the spots. It appears that, sj)eaking broadly, 

 the disturbance which produces the sun-spots begins in two belts 

 on each side of the sun's equator in a latitude of over 30^; these 

 belts or spot-zones then griidually move in towards the equator, 

 the sun-spot maximum occurring when their latitude is about 10°; 

 while the disturbance gradually and finally disappears at a latitude 

 of 8° or 10°, some twelve or fourteen years after its first appearance. 

 But two or three years before this disappearance, a new zone of 

 disturbance shows itself in the same latitude as its predecessor, 

 so that for a while, about the time of sun-spot minimum, there 

 are two well-marked zones of spots on each side of the sun's 

 equator; one pair near the equator, due to the expiring disturb- 

 ance which began some ten or eleven years ago; the other far 

 from the equator, and due to the newly arising out-burst, which 

 will reach its maximum in three or four years, and then pass 

 away like the former. 



There can be no doubt that the phenomenon is a very signifi- 

 cant one, but its explanation, like that of the periodicity itself, 

 is still to be found. 



Nor is the problem of the spots themselves yet fully solved. 

 Not that thei'e is any reasonable question that they are hollows 

 in the solar photosphere; but how they originate, how deep they 

 are, and what are the causes of their darkness, and the condi- 

 tion and temperature of the darkening substance ; these are 

 questions to which only uncertain answers can now be given. A 

 long and important series of observations upon the widening of 

 the lines of certain elements in the sun-spot spectra has been 

 made by Mr. Lockyer, and establishes clearly the fact that those 

 lines, of iro7i for "instance, which are conspicuously black and 

 wide in the sun-spots, are often just those which do iioi show 

 themselves conspicuously in the prominences; and moreover 

 both in spots and prominences the iron lines that do show them- 

 selves are most frequently those which closely coincide with lines 

 in the spectra of other substances. Singularly, also, and so far 

 quite without explanation, it appears according to his observa- 

 tions that at the sun-spot maximum those iron lines, which at 

 other times are conspicuous in spot spectra, entirely disappear. 



Perhaps I may be allowed to mention here a recent observation 

 ofmyownnpon these spot-spectra: with a high dispersion the 

 darkest part of the spot spectrum is found to be not continuous, 

 but made np of fine lines overlapping or almost touching each 

 other, with here and there a clear space left, like a fine bright 

 line. It means, I think, that the absorbing vapors which darken 



