2 



Mr. G. J. Stoney on the Physical 



[Recess, 



of such rays. That there are dark lines in the solar spectrum reveals to 

 us the fact that the surface of the atmosphere is cooler than the luminous 

 region beneath. But we may go further than this. Most gases are 

 colourless ; in other words, they do not scatter rays incident on them. 

 Neither do they reflect light from their surface. It follows, then, from 

 the laws which regulate the exchange of heat, that where such a gas is of 

 sufficient thickness to be opake in reference to any particular ray, it will 

 send forth the most intense ray of that particular refrangibility which it is 

 possible for a body of the temperature of the gas to emit. Hence that 

 there are lines in the solar spectrum of very different intensities is an 

 evidence to us that the surfaces of the atmospheres from which they have 

 their source are at different temperatures. It thus appears, upon a rough 

 view, that the upper layers of the atmospheres of sodium, magnesium, 

 and hydrogen are cooler than those of iron and calcium, and that these 

 again are cooler than the upper layers of the atmospheres of nickel, cobalt, 

 copper, and zinc. In this, then, we have evidence both that the atmo- 

 spheres of the several gases extend to different heights, and that the tem- 

 perature increases from the surface of the solar atmosphere downwards. 

 Again, such facts as that some of the iron lines are less dark than others 

 in their neighbourhood, that some of the copper lines are not noticeable, 

 prove that even before descending sufficiently far to have passed through 

 a stratum of these gases thick enough to be opake to these rays, we have 

 already arrived at a sensibly higher temperature. This temperature, in the 

 case of some of the lines of cobalt, copper, and zinc, appears to approach, 

 if it does not pass beyond, the temperature of the luminous clouds. 



3. Let us now direct our attention to the darker nucleus which lies 

 within the photosphere. It is known that a body, when surrounded on 

 all sides by an opaque envelope of others at its own temperature, will reflect 

 some of the incident light, if its surface be in any degree polished, and if 

 the body be not wholly transparent or wholly black. It will scatter others 

 of the incident rays if either its surface or its substance be such as would 

 not be wholly invisible if exposed to light brighter than that corresponding 

 to its temperature. And, lastly, it will emit rays in virtue of its own tem- 

 perature of such kinds and in such quantities that, along with those trans- 

 mitted, reflected, and scattered, they will make up a total which is definite 

 for each temperature. This is one of the established laws of the exchange 

 of heat. It follows from this that a body thick enough to be opaque which 

 emits much more feebly than others at a given temperature must reflect 

 incident rays better, or scatter them more copiously. It cannot in an 

 eminent degree do both. Unless the dark body of the sun is cooler than 

 the photosphere it is therefore, at least in those places which are exposed 

 to us as spots, either such that it scatters incident light abundantly, or it 

 has a highly reflecting surface. 



4, Before pursuing further our inquiry into the nature of the central 

 body of the sun, it will be convenient to enter upon the discussion of the 



