26 



Mr. G. J. Stoney on the Physical 



[Recess, 



of the layer, while in the stratum of luminous clouds it is able to hold its 

 ground with equal molecular motions, solely because the molecules are there 

 somewhat nearer together. It must therefore at the lower station be in a 

 state of almost inconceivable rarefaction ; and, from the laws of diffusion, 

 its density at any higher point can nowhere go beyond this. It appears, 

 therefore, almost in vain to expect to see bright lines in the solar spectrum. 

 If, however, any such exist*, they will probably be most readily detected 

 in light taken from near the margin of the sun's disk, where the bright- 

 ness of the region behind the luminous clouds is cut off, and where the 

 thickness of the stratum of attenuated gas which forms the bright lines is 

 increased by the oblique position of the spectator. 



3/. This rarefaction (which would be carried to an extreme in the case of 

 a gas, if any such exist, which extends into, but not beyond, the stratum 

 that is hotter than the luminous clouds) will also affect in a very consi- 

 derable degree those gases which do not spread far beyond it. Accord- 

 ingly the fainter lines in the solar spectrum either arise from such low- 

 lying gases in a state of great tenuity, in which case those lines only can 

 be visible in reference to which these gases are most opake, which will 

 therefore be the brightest of their artificial spectra ; or they arise from 

 constituents of the solar atmosphere which spread into the colder regions 

 above, in which case they can only be those lines in reference to which 

 these gases are highly transparent — such as are lines 50*48 and 53'52 of 

 the Calcium spectrum, and the lines 49'21 and 51*81 of the Nickel spec- 

 trum. It may perhaps be found that faint lines of this latter class will be 

 seen about equally distinctly in spectra formed of light taken from the 

 centre of the sun's disk, and in spectra formed of light taken from near its 

 margin. "When the light is taken from the centre these lines have the ad- 

 vantage of a brighter background to set them off ; when it is taken from 

 the margin they have in their favour the greater depth of Calcium or of 

 Nickel atmosphere which is looked through. But in the case of those 

 faint lines of the other class which originate in the lower strata of the sun's 



* I have several times thought I saw such a line, of wave-length 58*88, between the 

 more refrangible of the lines D and the next line recorded in Kirchhoff's map to the 

 left, almost in contact with this latter line. The appearance, however, may have arisen 

 from the adjoining part of the spectrum having been subdued by lines not marked on 

 Kirchhoff's map, and which a spectroscope of two equilateral flint-glass prisms could 

 not sufficiently make out. I sometimes received the impression that there were such 

 dim lines, but could not satisfy myself that they accounted for the bright line. Pos- 

 sibly there is also a bright line somewhere between the lines 1025*5 and 1027*7 of 

 Kirchhoff's scale, and another in the right hand of the two parts into which the space 

 between the lines D is divided by the Nickel line. Although it is on the whole im- 

 probable that the appearances are really due to bright lines, it would perhaps be worth 

 repeating the observations under more favourable circumstances, of which the most 

 important would be to admit only light from the margin of the sun's disk. If the sus- 

 pected bright line between the lines D should prove real, it is perhaps occasioned by 

 zinc. [For a continuation of this note, see the postscript, p. 57.] 



