1922.] The Ninth Indian Science Congress. T.S.C. 41 
searches of Kapteyn on the average mass of stars ! seem how- 
ever to indicate that there can be no great amount of such 
dark matter in the Stellar system. 
Stellar Distances. 
For an investigation of the arrangement of stars in space 
a knowledge of their distances is indispensable. The problem 
presents exceptional difficulties as the displacement in direc- 
tion when viewed from the opposite ends of a diameter of the 
earth’s orbit, is so minute even for the nearest stars. Sir 
David Gill once compared the problem to that of measuring 
“the angular diameter of a threepenny bit two miles away.” 
It is no wonder that very little progress could be made at first 
in this direction. 
The parallax of 61 cygni was first measured in 1838 by 
Bessel at the Kénigsberg Observatory. This was soon followed 
by the parallax measures for two other stars made by Hender- 
intensity of certain pairs of lines in its spectrum. He und 
it possible from this relationship to determine directly the 
i tion of its spectrum, at 

! British Association, Edinburgh, meeting 1921. 
2 M=m+5+5log a. 
