712 
PROFESSOR J. N. LOCKYER ON THE PHOTOGRAPHIC 
the condensations will be competent to show absorption phenomena, in 
consequence of the briglit continuous sjDectrum of the still disturbed lower 
levels of those atmospheres. 
Among the more important lines which will disappear at this stage will be those 
of iron, for the reason that there will be bright lines from the interspaces 
occupying the same positions as the dark lines produced by the absorption of 
the vapour surrounding the stones. 
The number of violent collisions per unit time and volume being further increased, 
we should expect the absorption of very high temperature vapours. 
I'he Hottest Stars. 
Ultimately, then, we should expect that the order of the absorbing layers will 
follow the original order of the extension of the vapours round the meteorites in the 
first condition of the swarm, and the lines seen bright in nebulae, wdiatever their 
origins may be, should therefore appear almost alone as dark lines in the hotter stars, 
and the hydrogen especially should have its lines broadened with each increase of 
depth in the atmosphere. The continuous absorption at the violet end of the 
spectrum will be at a minimum. If, when the hydrogen lines are thickest the swarm 
is not yet completely condensed, that is, if there be nebulous matter surrounding the 
central mass of vapour, a fine bright line will be seen down the centre of each 
dark one. 
Stars of Decreasing Temperature. 
When we consider the cooling condition, that is, what happens when the tem¬ 
perature of the mass of vapour is no longer increased by the fall towards the centre of 
meteorites composing the initial swarm, we should expect to find the phenomena 
indicated below. 
(Stage 1.)—The hydrogen lines will begin to thin out, on account of the 
diminishing depth of the absorbing atmosphere, and new lines will appear. 
As I have already pointed out,"^ the new lines will not necessarily be the same as 
those observed in connection with stars of increasing temperature. In the 
latter there will be the perpetual explosions of the meteoi'ites aftecting the 
atmosphere, whereas in a cooling mass of vapour we have to deal with the 
absorption of the highest layers of vapours. Those lines which will first 
make their appearance, however, vdll be the longest low temperature lines of 
the various chemical elements. 
(Stage 2).—The hydrogen lines wdll continue to thin out, and when the absorp¬ 
tion of the hotter lower layers makes itself felt the spectra will show the 
* ‘ Roy. Soc. Proc.,’ voL 45, p. 382. 
