286 SCIENCE PROGRESS 



its radiation is dispersed into a band of general darkening 

 longitudinally crossing the lines of the photospheric spectrum. 

 This band varies in intensity in different spots, being in large 

 spots generally very black, and in small spots of a decidedly 

 lighter tint, the difference being .due, no doubt, to the varying 

 quantity and depth of the materials contained in the spots. 

 If the materials in a sun-spot were simply opaque to light, 

 this variation in tint might be accounted for by the sparseness 

 or denseness with which the particles were packed together. 

 The effect produced, however, seems not, at least wholly, to be 

 due to the stoppage of light by opaque particles, because with 

 highly dispersive spectroscopes the dark band is seen to be 

 discontinuous, crossed by bright spaces. With very powerful 

 instruments the band of general darkening has been resolved 

 into a series of fine lines, and bands, or congeries of lines, 

 indicating a gaseous constitution. 



Besides this almost continuous band of general darkening, 

 the Fraunhofer lines are variously affected in a sun-spot. The 

 most general characteristic is, that, where they cross the spot, 

 they gradually widen though the penumbra to the umbra, in 

 which part of a spot the widening effect is most pronounced, 

 and then gradually narrow so as to present a sort of spindle 

 shape. Not all the Fraunhofer, or dark, lines of the solar spec- 

 trum are so affected when they cross a spot, but only some. 

 Taking the lines due to any given metallic vapour, iron for 

 instance, some lines will be widened in this manner and some 

 not. Some, too, will be weakened where they cross a spot. 

 And speaking of metallic lines in general, some will be obliter- 

 ated in a spot, some will be thinned, as, for instance, the lines 

 of hydrogen ; some will be reversed, if indeed it is a true reversal 

 of the line, and not rather a doubling of a line, which is single 

 in the photosphere. Other lines, though weakened in intensity, 

 will be widened ; other lines will extend as widened far beyond 

 the limits of the spot ; while others will have a sort of fuzzy 

 fringe attached to them, and be confined as spot bands to the 

 umbra. 



In addition to these variations in the line spectrum of a 

 spot, there is a whole series of bands made up of a great 

 number of lines forming flutings. These bands may become 

 so prominent in a big spot, as, even with a large dispersion, to 

 present the appearance of a local strengthening of the general 



