< 
“FEBRUARY 12, 1914] 
By making the observations desired by experts, every 
hour would be saved, and work of the greatest value 
accumulated. 
Astrophysics assumed prominence as a science about 
forty years ago, although it was foreshadowed by 
certain far-seeing astronomers, like the Herschels, 
G. P. Bond, Huggins, Draper, and others. One de- 
partment, the study of the light of the stars, was 
developed much earlier, originating in the Almagest, 
and its revision a thousand years later by Safi. These 
catalogues show that the relative brightness of the 
stars has not changed sensibly during the last two 
thousand years. Also, that the human eye has the 
same sensitiveness to different colours now as then. 
Stellar brightness was made a precise science by that 
great astronomer, William Herschel. His six cata- 
logues, two of which remained unknown for eighty 
years, give precise measures of the light of the three 
thousand stars contained in them with an accuracy 
comparable with recent work, 
In 1877, stellar photometry was taken up on a large 
scale at Harvard. Since then, more than two million 
photometric settings have been made. A station in 
Arequipa, Peru, permitted the southern stars to be 
observed on the same system as the northern stars. 
We have now, accordingly, measures of about eighty 
thousand stars, including all the seventh magnitude 
and brighter, many of the ninth magnitude, and some 
as faint as the thirteenth magnitude. The excellent 
work of the Potsdam Observatory gives measures of 
the light of fourteen thousand stars, including all 
northern stars of the magnitude 7-5 and brighter. 
The Potsdam and Harvard systems agree admirably 
if a correction is applied for the colour, or spectrum, 
of the stars. They should never be combined, or com- 
pared, unless this correction is applied. 
Stellar photography, originating in the work of 
George Bond in 1857, has revolutionised many depart- 
ments of astronomy. The great work of a chart of 
the entire sky, undertaken by the Paris Observatory 
in cooperation with several others, is a sad example of 
the danger of undertaking a work on too large a 
scale. Although several observatories have been con- 
tinually at work upon it for a quarter of a century, it 
has been predicted that at least fifty years must elapse 
before it is completed, and no positions of any southern 
stars have yet been published. In striking contrast 
to this is the early completion of the Cape Photo- 
graphic Durchmusterung, which gives the positions 
and magnitudes of nearly half a million stars south of 
—19°. It illustrates the results of the happy combina- 
tion of skilful planning with routine organisation, 
conducted on a very large scale. The extension of 
this work to the north pole is now being planned, but 
with the additional condition that the colour index, as 
well as the photographic magnitude, will be deter- 
mined. The former will be found by photographing 
the stars by means of their yellow or red, as well as 
their blue, light, the difference in the magnitudes 
giving the colour index. 
Much might be said of the numerous applications of 
photography to the determination of stellar magnitude. 
The 60-in. reflector of the Mount Wilson Observatory, 
using exposures of several hours, has succeeded in 
photographing stars as faint as the twentieth mag- 
nitude. An international committee, with members 
from England, France, Germany, Russia, Holland, 
and the United States, has adopted a scale of mag- 
nitudes based on two investigations made at Harvard. 
One of these was made with the meridian photometer, 
and the other is an elaborate investigation by Miss 
H. S. Leavitt of the photographic magnitude of 
seventy-six stars near the north pole. A standard 
scale is thus provided from the first to the twentieth 
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magnitude. We may say from the minus twenty-sixth 
to the twentieth magnitudes, since accordant results 
for the light of the sun have been obtained by Profs. 
W. H. Pickering and E. S. King. For many pur- 
poses photography may well replace visual photo- 
metric measures, since for stars brighter than the 
fifteenth magnitude photographs may be taken with 
yellow light. 
One of the principal uses of measures of the light 
of the stars is the study of the variables, or those in 
which the brightness is not constant. A bibliography 
of these by Miss Cannon is recorded on about forty 
thousand cards. The number of known variables is 
now about forty-five hundred, of which three-quarters 
have been discovered by photography at the Harvard 
Observatory. There are several kinds of variable 
stars. Variables of long period undergo changes 
which repeat themselves somewhat irregularly in a 
period of several months, and at maximum are often 
several thousand times as bright as at minimum. 
The most useful work that an amateur can do with a 
small telescope is the observation of those objects. An 
important work undertaken by members of the British 
Astronomical Association has been the observation of 
variable stars. During the last thirteen years they 
have accumulated twenty thousand such observations, 
all reduced to the same scale, which is that of the Har- 
vard photometry. Similar work in the United States 
has accumulated ten and sixteen thousand observa- 
tions respectively in the last two years. 
Variables of short period complete their changes in 
a few days, or hours. Prof. Bailey has found five 
hundred such objects in the globular clusters. In one 
of these clusters, Messier 3, out of a thousand stars 
one-seventh are variable, all have a period of about 
half a day, and their periods are known within a 
fraction of a second. Their light changes so rapidly 
that in one case it doubles in seven minutes. It is a 
strange thought that out of a thousand stars, looking 
exactly alike, there should be a hundred little chrono- 
meters keeping perfect time, and the rate of which 
is known with such accuracy. About a hundred and 
fifty variables belong to the Algol class, in which the 
light is uniform for a large part of the time, under- 
going a sudden diminution at regular intervals. This 
is due to the eclipse of two bodies, one darker than 
the other, revolving around their common centre of 
gravity. An elaborate theoretical study of this 
problem has been made at the Princeton Observatory, 
and, from the photometric and photographic mag- 
nitudes made at Harvard and elsewhere, the dimen- 
sions of a large number of these systems have been 
determined. 
Photography still can scarcely compete with other 
methods where the greatest accuracy is desired, as, for 
instance, the measures with the polarising photometer 
by the late Oliver C. Wendell. The masterly use of 
the selenium photometer by Prof. Stebbins gives re- 
sults for bright stars of still greater accuracy, while 
the experiments in Germany with the photo-electric 
cell by Rosenberg and Guthnick give results which 
promise to revolutionise our present methods. The 
principal source of error appears to be the varying 
transparency of the air. The trial of the instrument 
in a location where the air is exceedingly clear and 
steady for long periods is greatly to be desired. 
During the last twenty-five years photographs have 
been obtained by the Harvard Observatory in order 
to furnish a history of the stellar universe. Two 
similar 8-in. photographic doublets have been used, 
one mounted at Cambridge for the northern, and the 
other at Arequipa, for the southern stars. With each 
of these instruments about forty thousand photographs 
have been taken. The total weight of these plates is 
