710 
PROFESSOR J. K LOCKYER OR THE PHOTOGRAPHIC 
Group III.—Line absorption predominant with increasing temperature. The 
various species will be marked by increasing simplicity of spectrum. 
Group IV.—Simplest line absorption predominant. 
Group V. — Line absorption predominant, with decreasing temperature. The 
various species will be marked by decreasing complexity of spectrum. 
Group VI.—Carbon absorption predominant. 
Group VII.—Extinction of luminosity. 
The fundamental difference between this and other classifications is that it demands 
the existence of bodies of increasing as well as bodies of decreasing temperatures. 
We have, tlierefore, to inquire how fen- this condition is satisfied by the mass of 
new facts at our disposal. This involves the consideration of some points in connec¬ 
tion with the meteoritic hypothesis to which brief reference alone has been made in 
my previous communications. 
The Complex Origin of the Spectra of Nehidce. 
On the hypotlresis, the bright lines seen in the nebulas should have three origins. 
(1.) The lines of those substances which occupy the greatest volume (or largest 
area in a section); in other words, the lines of those substances which are 
driven furthest out from the meteorites and occupy the interspaces, Avhen 
possibly they may be rendered luminous by electricity. Chief among these, 
from laboratory experiments, we should expect hydrogen, and next, from the 
same experiments, we should expect gaseous compounds of carbon. 
(2.) We are justified in assuming that the jnost numerous collisions will be 
partial ones—grazes—sufficient only to produce comparatively slight rises 
in temperature. The nebula spectrum, so far as it is produced by this cause, 
will therefore depend upon the phenomena produced in greatest number, 
and we may therefore expect to find the low temperature lines of various 
metallic substances. 
(3.) In addition to the large number of partial collisions there will be a relatively 
small number of end-on collisions, producing very high temperature,* and, 
so far as this cause is concerned, there will be some lines produced which are 
associated with very high temperatures. 
Combining these conclusions, in the spectra of nebulae wm should expect to find 
evidence of 
Hyd rogen. 
Compounds of carbon. 
Low temperature metallic lines and flutings. 
Lines which are only produced at very high temperatures. 
* ‘ Roy. Soc. Proc.,’ vol. 4.3, p. 150. 
