48 



Mr. G. J. Stoney on the Physical 



[Recess, 



sun, should be all of them more intense than the corresponding lines of the 

 sun's spectrum. Moreover, many substances which by reason of the large 

 mass of their molecules are unable to stand in the sun the low temperature* of 

 the clouds of the photosphere,and are therefore confined to the regions within, 

 are able, on a star which attracts with less force or whose centre is farremoved, 

 to pass through this obstacle and show themselves in the atmosphere above. 

 Finally, such stars will be ruddy. The sun himself is a somewhat ruddy 

 star, as may be seen by a glance at Tables II. and III. Both ends of 

 his spectrum are subdued by lines. The yellow, orange, and scarlet are 

 nearly as bright as they came from the photosphere ; the green is sensibly 

 shaded over by lines ; the blue suffers somewhat more ; the indigo about 

 G and the crimson beyond B, very much ; and the violet from G to H to 

 such an extent, that it is difficult to find a spot where the full light of the 

 photosphere appears to penetrate. The chemical rays beyond the violet 

 are progressively more and more enfeebled as their wave-lengths shorten ; so 

 much so, that the fluorescent spectrum from several artificial sources is longer, 

 and from some much longer, than the sun's. A similar deficiency seems 

 to exist at the other end in that prolongation of the solar spectrum beyond 

 A, which Sir David Brewster has dimly seen and succeded in figuring. 

 Now every encroachment upon the spectrum will be more marked when 

 very dark lines become numerous, that is, in stars hotter, or of smaller 

 mass ; and if the lines themselves are pretty evenly distributed, it will sub- 

 due the different colours in proportion to their refrangibilities f. Ruddy 

 stars, therefore, either have a less mass than our sun, or are more dilated 

 by heat throughout the regions beneath the photosphere. 



82. The consequences of the two other alternatives, of a star's mass 

 being greater than the sun's, or of the temperature within the photosphere 

 being less fierce, so that these regions are of less bulk, will be plain now. 

 In such stars, some of the substances which range through the part of the 

 sun's atmosphere above the photosphere are imprisoned within that lumi- 

 nous shell. Others of them, such as iron, calcium, and those of a like 

 vapour-density, can only hold their ground while at a higher temperature, 

 and would show faint though numerous lines in the spectrum. A few, 

 such as sodium, magnesium, the substance that causes the line B (if this 

 ray be of solar origin), and above all, hydrogen, would perhaps still con- 

 tinue dark. The lines of hydrogen, from its incomparably small vapour- 

 density, would be so much the last to yield, that there is probably no star 

 with gravity so intense as to produce any sensible impression upon them. 

 And accordingly, in all very white stars which have been examined, these 

 four lines stand out in extraordinay prominence. 



83. It is now no longer a mystery why solitary stars are either white or 

 of a red or yellow tinge. In all those cases in which the dilatation of the 

 central parts by heat is so proportioned to the mass of the star as to render 



* A variation of this temperature from star, to star is another circumstance which 

 must tell on the composition of the outer atmosphere. t See § 52. 



