AucusT 26, 1915| 

NATURE 
717 

few and are difficult to measure, and in all classes 
it is only possible to measure the displacements of the 
lines of the bright stars. Even if we anticipate im- 
provements in the art of spectrography, it would seem 
impossible to obtain spectroscopic data in the form 
required for more than twenty or thirty thousand 
of the brighter stars. Therefore, although spectro- 
scopy will be a useful ally, its help is limited. 
Let us now collect the data which are at the 
astronomer’s disposal for finding the distance of the 
more distant stars. The most important datum is the 
star’s proper motion. This is compounded of the 
reflex of the sun’s motion and of the star’s own proper 
motion, which latter may be eliminated by a process 
of judgment by assuming that the star is an average 
member of its group and spectral class, or that it 
belongs to one or other of Kapteyn’s two drifts. 
Although in individual cases these indications may be 
very erroneous, yet in the gross they are permitting 
astronomers to classify the stars into manageable 
groups. 
What is wanted is a better knowledge of the proper 
motions of the stars. Unfortunately at present these 
are not well known except for perhaps 10,000 of the 
brighter stars. Hitherto, the finding of the proper 
motions of the stars has been slow, arduous, and 
expensive work. At least ten meridian observations, 
spread over half a century, were essential, and each 
meridian observation cost about 20s., and meridian 
observations can only be made of the brighter stars— 
of perhaps 100,000 out of 1,000,000,000 stars now with- 
in the reach of the largest telescopes, or of one star in 
every 10,000. This proportion is altogether too one- 
sided. Hence astronomers hailed the advent of the 
photographic dry plate. An organisation for a Carte du 
Ciel was formed, in which our first president, the late 
Sir David Gill, was one of the chief promoters, and 
this scheme has now been at work for twenty-eight 
years; but, so far, the first Carte is far from complete. 
When completed in ten or twenty years’ time, we may 
expect it to furnish us with precise positions of some 
3,000,000 stars (or of about 1 star in 300, still a very 
small proportion). We will not know the proper 
motions of these stars. To achieve that, another 
Carte du Ciel must be prepared, so that we must 
expect another half-century to elapse before we are in 
possession of these 3,000,000 proper motions. Again, 
the labour, and with it the cost, involved is enormous, 
and will probably be in the neighbourhood of tos. a 
star. 
The drawback to these two methods of obtaining 
proper motions is the necessity for defining the exact 
position of each star at different epochs, whilst what 
we want is not its exact position, which is difficult to 
define, but its change of position—that is, its proper 
motion. At the beginning of this century it had been 
suggested that there was no necessity to measure the 
places of all the stars on photographic plates, but that 
if pairs of plates were examined in the stereoscope, 
those stars which had moved relatively would stand 
out in relief; alternatively, that if pairs of plates 
were superimposed, those stars which had moved by 
proper motion would easily be picked out. These sug- 
gestions were tried, and led to the discovery of a few 
proper motions, but the method was not workable on 
a large scale, mainly because of fatigue or strain upon 
the eyes. A third alternative was discovered by Dr. 
Pulfrich, of Jena, and described by him.as a blink 
method. By this method the pair of plates to be 
examined is placed side by side, like the pictures in a 
stereoscope, but they are examined with one eye 
through an optical and mechanical arrangement which 
rapidly lets the eye rest first on one plate and then 
on the other, so that in one second the eye has looked 
at each plate separately three or four times. This 
.NO. 2391, VOL. 95] 

' with in groups. 

blinking makes the eye wonderfully sensitive to the 
slightest shift upon the plates. If one star relatively 
to its neighbours has shifted a hundredth of a milli- 
metre upon a Carte du Ciel plate, the change is not 
only unmistakable, it is obtrusive. This blink-method 
revolutionises astronomy of position as regards the 
stars. Both with the meridian observations and the 
Carte du Ciel measurements, each star had to be dealt 
with separately. In the blink method the stars are dealt 
Indeed, one can say that it is easier 
to deal with 1000 stars by the blink method than with 
one by the other methods. All that the blink method 
requires is pairs of plates separated by as long inter- 
vals as possible. A few weeks ago Mr. Hough (H.M. 
Astronomer at the Cape) placed in Mr. Voute’s and my 
hands a pair of plates with a time interval of nearly 
twenty-three. years. There were about 10,000 stars 
on the two plates; in a few hours we were able to 
announce that only twenty of these showed proper 
motion—the rest were fixed stars—and we were able 
to find the proper motions of many stars which were 
so faint that even the great Carte du Ciel would not 
have included them. Since then further pairs of Cape 
plates haye been placed at my disposal with intervals 
of sixteen to eighteen years; the results confirm the 
earlier experience. We can therefore clearly state that 
astronomers have now a weapon of attack which will 
in the course of time reveal to them, without arduous 
or expensive labour, the proper motions of all classes 
of stars from the brightest to the faintest. This will 
lead to a knowledge of the structure of the sidereal 
universe which a few years ago seemed unattainable. 
The immensity of the task when tackled by the old 
methods seemed so great, and the consequent delay so 
inevitable, that Kapteyn proposed that astronomers 
should concentrate their attention on certain selected 
areas which might be taken as representative samples 
of the whole sky. 
A rude analogy will perhaps help us. The old way 
was something like studying the condition of England 
by means of a ‘“‘Burke’s Peerage” or a ““Who’s 
Who.” Kapteyn proposed as better a limited number 
of selected areas, some urban, some rural; but the 
blink method will easily cover the whole area and 
permit an exact census to be taken. — ¢ 
The present state of astronomical science is one of 
great activity, but I have only time to make some 
brief references. The activities of the Union Observa- 
tory, an institution originally started by our associa- 
tion, call for some mention. The late Mr. Frankkxlin- 
Adams planned a photographic chart of the whole sky, 
and more than half of the plates were taken at the 
Union Observatory. These were forwarded to the 
Astronomer Royal at Greenwich, and are undergoing 
examination. Some of the first results of this exam- 
ination have been published in the Memoirs of the 
Royal Astronomical Society. Counts of the stars on 
these plates have been made by Messrs. Chapman and 
Melotte,t from whom the following figures are 
taken :— 
Galactic Plates taken Plates taken 
Latitude in S. Africa in England 
ool tomas 988,000 515,000 
16 to 30 616,000 383,000 
31 to 50 406,000 230,000 
Eto” <90 307,000 145,000 
This little table invites two comments—one is that 
the purity of the atmosphere has resulted in many 
more stars (nearly twice as many) being found on the 
plates taken at the Union Observatory; the other 
that the richness of the plates decreases more or less 
uniformly as the galactic plane—the Milky Way—is 
left. Chapman and Melotte also give this table, show- 
4 Mem. RAS., x. = 
