THE SPOTS ON THE SUN. 151 



the universe, which has been built around him and for him, and with 

 reference to him as the supreme object. This led to the belief in the 

 inhabitability of worlds ; for, if stars and planets were not for man to 

 live on, what can they be good for ! And, hence, solar theories were so 

 framed that it might be possibxe to conceive of the sun as the dwell- 

 ing-place of man. The nucleus or body of the sun was assumed to be 

 opaque and solid, or, at all events, that it had a solid shell correspond- 

 ing to the earth's crust. Surrounding the spherical nucleus at a cer- 

 tain distance above it, there was supposed to be a first atmosphere 

 which may be compared to the earth's atmosphere when the latter is 

 occupied by a continuous layer of opaque, reflecting clouds. Above 

 this first layer, and more or less distant from it, there was held to be a 

 second atmosphere which is luminous, and answers to the photosphere, 

 or the visible periphery of the solar orb. 



On this view a solar spot resxilts from a rent in the atmospheric 

 layers, by which the dark nucleus becomes visible, forming the umbra. 

 The rupture of the two atmospheres, it was supposed, might be caused 

 by volcanic action, or by " gaseous matter formed from time to time 

 at the surface of the dark nucleus, the high temperature of which 

 causes its deflagration." Again it was said : " It may happen that the 

 opening is wider in the cloudy atmosphere than in the luminous envel- 

 ope or photosphere, in which case the dark nucleus alone would be 

 seen, and we should have a spot without a penumbra. Or the rupture 

 of the first gray envelope becoming closed before that of the photo- 

 sphere, would have for effect to shut out the view of the dark globe, 

 and we should have a penumbra without a nucleus." 



The later tendency is to abandon the notion of a dark nucleus. In- 

 deed, the explanations of the spots now most in favor recall that of 

 Galileo: he suggested a floating scum, while all the late physicists 

 hold that the spots are due to the agency of precipitated clouds. 



Kirchhoff, whose honor it is to have first applied spectrum analysis 

 to the study of the sun, and discovered its chemical elements, main- 

 tains that the visible portion of the sun, the surface which constitutes 

 the photosphere, is a solid or liquid sphere in a state of incandescence. 

 Its temperature is very high, and it is surrounded by a dense atmos- 

 phere formed of the elements which constitute the incandescent globe 

 itself, whose extremely high temperature maintains them in a state of 

 vapor or gas. The lines of the solar spectrum, instead of being bright 

 and variously colored, are dark, which proves that the light has passed 

 through a medium of absorption. Kirchhoff's view is, that the light 

 emanates from the solid or liquid photosphere, and is filtered of its 

 colors or has its lines reversed in passing through the sun's atmos- 

 phere. 



He explains the spots by supposing that, from some unknown cause, 

 certain parts of the sun's surface undergo a temporary cooling, by 

 which clouds are condensed above that intercept the rays and then ap 



