Juxe 3, 1915] 
NATURE 
387 

compared with the heliometer has, however, lost much 
of its force; for, as I hope to show next, the highest 
accuracy attainable with the heliometer can be secured 
much more easily with a photographic telescope. 
The application of photography to the determination 
of stellar parallax was first made by Pritchard in 
Oxford between 1887 and 1889. He took a large 
number of photographs and measured on them the 
angular distance of the star which he was consider- 
ing from four of its neighbours. In this way he deter- 
mined the parallax of five stars. He began this 
work late in life, and it was left for others to develop 
the photographic method and find what accuracy 
could be attained with it. At first sight it seems very 
easy, but experience shows that there are a number 
of small errors which can creep in and vitiate the 
results, unless care is taken to avoid them. 
It has gradually become clear that with a few 
simple precautions and contrivances, a greater accu- 
racy can be reached in the determination of parallax 
by photography and with much less trouble than. by 
any other method. Between 1895 and 1905, several 
astronomers succeeded in obtaining from a few plates 
results as accurate as could be obtained from many 
nights’ observations with the heliometer by the most 
skilled observers. In the last five years a large num- 
ber of determinations have been made. In 1910 
Schlesinger published the parallaxes of twenty-five 
stars from photographs taken with the 4o-in. refractor 
of the Yerkes Observatory, and in 1911 Russell pub- 
lished the parallaxes of forty stars from photographs 
taken by Hinks and himself at Cambridge. The 
opinion expressed by Gill on these observations (M.N., 
vol. Ixii., p. 325), was that but for the wonderful 
precision of the Yerkes observations, the Cambridge 
results would have been regarded as of the highest 
class. The facility with which the Yerkes results are 
obtainable is expressed very tersely by Schlesinger— 
“the number of stellar parallaxes that can be deter- 
mined per annum, will in the long run be about equal 
to the number of clear nights available for the work.” 
With the heliometer at least ten times as much time 
would have been required. During the last year two 
further instalments of the results of the Yerkes Ob- 
servatory have been published by Slocum and Mitchell, 
giving the parallaxes of more than fifty stars. It 
might be thought that the high accuracy attained by 
them is largely attributable to the great length of the 
telescope. From experience at Greenwich, I do not 
think this is the case, and believe that similar results 
are obtainable with telescopes of shorter focal length. 
As several observatories are now occupied with this 
work, we may expect that the number of stars the 
distances of which are fairly well known will soon 
amount to thousands, as compared with three in 1838, 
about twenty in 1880, about sixty in 1900, and now 
perhaps two hundred. 
The stars the distances of which have been measured 
have generally been specially selected on account of 
their brightness or large proper-motion. I:ach star 
has been examined individually. Kapteyn has sug- 
gested that instead of examining stars singly in this 
way, photography gives an opportunity of examining 
all the stars in a small area of the sky simultaneously, 
and picking out the near ones. The method has been 
tried by Kapteyn and others—among them Dr. Ram- 
baut. The idea is very attractive, because it examines 
the average star and not the bright star or star of 
larger proper-motion. It is liable, however, to some 
errors of systematic character, especially as regards 
stars of different magnitudes. Comparison of the 
results so obtained with those found otherwise will 
demonstrate whether these errors can be kept suffi- 
ciently small by great care in taking the photographs. 
Until this is done no opinion can be expressed on 
NO. 2379, VOL. 95| 

the success of this experiment, which is worth careful 
trial. 
The question may be asked, How near must a star 
be to us for its distance to be measurable? I. think 
we may say ten million times the sun’s distance. 
This corresponds to the small angle o-o2" for the 
parallax. If a star’s parallax amounts to this, there 
are, I believe, several observatories where it could be 
detected with reasonable security, though we shall 
know more certainly by the comparison of the results 
of different observations when they accumulate. 
You will readily imagine that an accurate know- 
ledge of the distances of many stars will be of great 
service to astronomy. There are ample data to deter- 
mine the positions, velocities, luminosities, and masses 
of many stars if only the distances can be found. 
Thus we know the distance of Sirius, and we are able 
to say that it is travelling in a certain direction witn 
a velocity of so many miles per second: that it gives 
out forty-eight times as much light as the sun, but 
is only two and a half times as massive. The collec- 
tion and classification of particulars of this kind is 
certain to give many interesting and perhaps surpris- 
ing results. But it is not my purpose to deal with this 
to-night. The task I set before myself in this lecture 
was to give an idea of the difficulties which astro- 
nomers have gradually surmounted, and the extent to 
which they have succeeded in measuring the distances 
of the stars. 
UNIVERSITY AND EDUCATIONAL 
INTELLIGENCE. 
Campripce.—The Rev. T. C. Fitzpatrick, president 
of Queens’ College, has been elected Vice-Chancellor 
for the coming academic year. 
The council of the Senate has announced that the 
Board of Education has agreed to make a grant in 
aid of the medical departments of the University, and 
that the amount to be received on account of the 
present year is 5873/.; it proposes that a new com- 
mittee, to be called the Medical Grant Committee, 
should be appointed for the purpose of administering 
the annual grant. 
Mr. H. Scott, of Trinity College, has been appointed 
curator in entomology for five years from March last. 
A grant of sol. has been made from the Balfour 
Fund to enable Mr. G. Matthai, of Emmanuel College, 
to visit America in furtherance of his researches on 
the comparative anatomy and classification of the 
Madreporaria. 
GiasGow.—By an Order in Council dated May 27, 
tgt5, the King has reappointed Sir Donald MacAlister 
and Mr. Otto Beit, and has appointed Mr. R. E. 
Prothero, to be members of the governing body of the 
Imperial College of Science and Technology for a 
term of four years from June 1, I9I5. 
Lonpon.—Owing to Prof. Brodie’s services being 
required for military purposes, the course of advanced 
lectures in physiology arranged to be given by him 
at King’s College (as announced last weelx) will not 
be delivered. 
Oxrorp.—The report of the delegates of the Uni- 
versity Museum for 1914, together with the depart- 
mental reports of the professors and readers teaching 
within the Museum, has just been published. In the 
general report attention is directed to the departure 
of large numbers of the teaching and service staff, 
and also of research workers, for military duties. 
This has affected both the Museum itself and also 
the several departments. Mention is made of the 
billeting of the 9th Battalion of the Hampshire Regi- 
ment for a night in some of the laboratories; of the 
presentation to the University of Mr. Hope Pinker’s 
