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ASTRONOMY: B. N. RUSSELL 
single plate, but as reference points for proper motion when comparing 
two plates of different epochs. The observations of these reference stars 
must at present be made with meridian circles; but the proposed methods 
for determination of absolute positions of the stars by photography- 
deserve careful study and trial. 
Pending the completion of such a program, the investigation of the 
proper motions of faint 'optical' companions of bright stars, such as 
has been made by Comstock,^^ furnishes our best source of information 
concerning the proper motions of faint stars, but is complicated by 
systematic errors in the early measures. A survey of the whole heavens 
for stars of large proper motion is very desirable. In this case it is 
legitimate to treat the general ' background' of stars as at rest, and the 
observations can be very rapidly made, with the blink microscope or 
similar appliances. Early plates are probably already available for 
almost, if not quite, the whole of the heavens. Such an investigation 
is likely to yield important information concerning the stars of very 
small absolute luminosity — as is shown by Barnard's^* and Innes's^^ 
recent remarkable discoveries — and should be extended to the faintest 
accessible stars. 
Comparison of measures of plates taken at different epochs (still 
treating the bulk of the stars as fixed) will yield much information about 
proper motions of moderate size. This has already been done on an 
extensive scale with plates of the Astrographic Catalogue. 
Special investigations should be made to determine at an early date the 
proper motions of all stars belonging to certain interesting classes for 
which early determinations of position are available — for example, 
binaries, variables, and stars having peculiar spectra. 
7. The study of the radial velocities of the stars is intimately associated 
with that of the proper motions. The determination of radial velocities 
with the slit spectroscope has been brought to a high degree of perfec- 
tion, but the separate investigation of each one of the many thousands 
of stars which are now accessible would involve an enormous amount 
of labor. The development of some method by which radial velocities 
could be determined en masse with the objective prism would be a 
great boon. If some absorbing medium giving sharp and well dis- 
tributed lines in the blue and violet could be found, the problem would 
become simple; and other solutions are doubtless possible. 
It is also desirable that some method be devised for obtaining, at 
least approximately, the radial velocities of stars possessing spectra 
with very diffuse lines. At the present time, no radial velocities have 
been published for some of the very brightest stars, on this account. 
