xxvjn.j SPOT HISTORY. 407 



attain its full size in five or six days, although exceptionally large 

 ones occasionally occupy a longer time in their first development. 

 If no remarkable increase is noticed in a spot within two or 

 three days from its birth, it will in all probability never attain 

 to any considerable size. The solar surface has repeatedly been 

 examined with the greatest care, in regions where considerable 

 spots have broken out on the following day, without detecting 

 any marked disturbance or other sign announcing a probable 

 outburst. No satisfactory exceptions to this have as yet been 

 noticed." 



Professor Sporer, on the other hand, 1 is inclined to think 

 that spots form ooly on the more luminous parts of the photo- 

 sphere, that is in regions of higher temperature ; and indeed I 

 gather that in his view an up-rush begins the disturbance. But, 

 even if his observation be correct and it differs, as will be seen, 

 fundamentally from those of Perry (see above quotation), the 

 greater illumination of the photosphere may be explained by 

 supposing that the great down-rush which produces a spot may 

 be preceded by slighter falls which disturb the photosphere 

 without giving rise to large masses of absorbing vapour or the 

 appearance of facula?. 



With regard to the absorbing vapour itself it must not be 

 forgotten that it must be at a very high temperature. 



Professor Hastings of Baltimore 2 thinks it is something like 

 smoke, but this is negatived by the spectrum observation made 

 and recorded years ago. On this point Professor Young has 

 lately written as follows : 



" With a high dispersion the darkest part of the spot spectrum is 

 found to be not continuous, but made up 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 

 vapours which darken the interior of the spot are wholly gaseous, 

 and tends to disprove the idea that they are mostly of the nature of 



1 Nature, Nov. 18, 1886, page 72. 



2 American Journal of Science, Jan. 1881. 



