702 
in competent hands. It is directly appli- 
cable to one of the great problems of as- 
tronomy—the determination of the direction 
and rate of the sun’s drift through space. 
From the proper motions of the the stars, 
furnished by the methods of the older as- 
tronomy, the direction of the sun’s motion 
ean be deduced, and, under certain as- 
sumptions as to the stars’ distances, the 
rate of motion; butit is evident that the 
latter element of the problem must be sub- 
ject to very considerable uncertainty. With 
the spectroscope velocities are directly 
measured in miles per second. The two 
methods may be combined. It is probable 
that the most accurate determination of the 
direction of the sun’s drift can be obtained 
by preparing proper motions, while the 
most accurate value of the velocity is that 
given by the spectroscope. Thus by the co- 
operation of the two branches of astronomy, 
there is measured in space a base line of 
constantly increasing length for a great 
sidereal triangulation. At present the ma- 
terial afforded by spectroscopic observation 
is not sufficient for this great work. The 
observations must be treated statistically, 
and statistical methods can be applied suc- 
cessfully to only a large mass of data. What 
is now needed, therefore, is observations of 
more stars, 7. ¢., fainter stars, and the Ger- 
man government is building a large tele- 
scope for the observatory at Potsdam 
(where photography was first applied to 
this class of observations), in order that 
the work may be continued. There is 
room, however, for the employment of 
other large telescopes in the same field. 
The multiplication of observations for this 
purpose is no more to be deprecated than 
the multiplication of observations for the 
exact determination of star places. 
Solar physics, from which the wider sci- 
ence of astrophysics has been evolved, 
offers problems so numerous and so compli- 
eated that I cannot even mention them, 
SCIENCE. 
[N.S. Von. VI. No. 151. 
still less enter into a discussion of their 
bearing on other branches of knowledge. 
And what can I possibly say of their im- 
portance? The sun is to us the grandest 
of material objects. It is the source of 
practically all our light and heat; of prac- 
tically all our mechanical power; abso- 
lutely the support of all our lives. What 
wonder that we seek for knowledge of its 
nature by all the ways that we can find! 
These ways are opened through astrophys- 
ical research. In few of the inquiries that 
I have referred to can the method of light 
analysis be dispensed with. In most of 
them it offers the only chance of success. 
I have time to mention only one new 
method of solar research. The most no- 
table contribution to solar physics within 
the last few years has been the invention of 
the spectroheliograph by Hale and Des- 
landres. With this instrument photographs 
of the sun are taken by strictly monochro- 
matic light, which may be chosen from any 
part of the spectrum. If the part selected 
is the middle of the K line, the picture 
essentially represents the distribution of 
calcium vapor on the disk of the sun, and 
the presence of other elements is ignored. 
This is, in fact, the line usually chosen, 
partly on account of the conspicuous réle 
played by calcium in solar phenomena, and 
partly for other reasons, which it is not 
necessary to state. The possibilities of the 
method are obvious. By an ingenious 
modification of his instrument Hale now 
photographs on a single plate the Sun cov- 
ered with all its spots and facule, and sur- 
rounded by all its prominences; and all 
this is done in a few minutes, in full day- 
light! Could the corona be added, the 
triumph would be complete ; but the corona 
yet remains unconquered in its stronghold, 
though the attack is being vigorously 
pushed. 
No branch of observational astronomy 
seems to be in so backward a state as the 
