148 
SEE. 
It is significant that what progress has been made recently 
in explaining the variability of stars is chiefly by the theory 
of tides; and some new support is given this theory by 
Campbell’s discovery of dark companions to several well- 
known variables. 
In the line of planetary observations mention may be 
made of a series of new determinations of diameters of the 
principal planets undertaken by Professor T. J. J. See, of the 
United States Naval Observatory. By the use of a color 
screen the outstanding color of the telescope is removed and 
the image steadied and sharpened at the limb. 
Observations have offered a fair test of the screen in the 
case of Neptune, Uranus, Jupiter, Saturn, Venus, and Mer¬ 
cury, but the final values have not yet been obtained ex¬ 
cept in the case of Jupiter, Saturn, and Venus. It is thought 
that this work, when completed, will afford a good set of 
diameters of the planets, all made by the same observer, 
under like conditions. Professor See contemplates deter¬ 
mining the oblateness of all the planets, and also their con¬ 
stants of irradiation, so that their figures, absolute diameters, 
and densities may be deduced. 
Mention may be made, in this connection, of progress look¬ 
ing to the installation of a new clock system at the Naval 
Observatory. The effects of varying atmospheric pressure 
are to be avoided by inclosing the clock in a hermetically 
sealed case, kept in a vault at constant temperature—an 
arrangement which has been in use a good many years in 
several of the great European observatories. If successful, 
this will afford a new basis for fundamental work on the 
principal fixed stars, and materially elevate the standard of 
American astronomy. 
Professors Brown and Updegraff have together initiated 
this much-needed improvement, and good results may be 
expected to follow from it in due time. Professor Pees, of 
Columbia university, has finished his determination of the 
constant of aberration, from latitude observations extending 
over six years, and reaches as his final figure 20".464. 
