1922.] The Ninth Indian Science Congress. 1L.8.C. 37 
tions have contributed largely towards a successful attack on 
the problem of the Structure of the Universe. These 
(1) The general introduction of photographic methods 
consequent to the invention of the “ dry plate.” 
and (2) The application of the spectroscope to the study of 
the heavenly bodies. 
oo 
ad 
7. The immense power of the photographie method rests 
on some of its essential properties. First, the impressions can 
be accumulated—a fact of considerable importance when faint 
objects are concerned. The photographic plate enables us to 
store up the feeble light impressions received through the tele- 
scope, so that even a small instrument, in these days of dry 
plate, acquires ‘‘a new dimension”’ and can, in some respects, 
produce results quite as efficiently as a large one. 
e second important factor is that by the photographic 
method large regions of the heavens can be recorded at the 
same exposure. Prof. Barnard, Max Wolf and others, have 
thus studied the structural peculiarities of the Milky Way and 
other special regions which could never have been revealed by 
the visual method. 
e accuracy and the facility in manipulation have all 
added to the importance of this new method. In fact it has 
enabled progress to be achieved by leaps and bounds especially 
in directions where only slow headway could be made before. 
8 
formation regarding its temperature, physical condition and 
chemical constitution. By the observation of the lines in the 
Spectra of stars we learn besides, how the stars may be classi- 
fied, how fast they are moving and in several cases how intrin- 
sically bright they are. I shall deal with these points later on 
in this address. The spectroscope in the Slitless form (the 
. 
objective Prism) has enabled Prof. Pickering and his col- 
Secchi’s divisions into Types and is now generally adopted by 
all investigators. The main classes in the Harvard ee 
are designated by the capital letters OBAP. 
while there are a number of sub-classes occupying intermediate 
positions in this scale. 
The sequence is in order 0 
being the hottest. The designations are 
f temperature, the O and B stars 
not in the same order 
