386 

Simultaneously with these determinations of the 
distance of a Lyre and 61 Cygni, the distance of 
a Centauri, one of the brightest of the southern stars, 
was found by Henderson from observations of zenith 
distance made by him at the Cape between April, 
1832, and May, 1833. He learnt just before the ter- 
mination of his residence at the Cape that this star 
had a very large proper-motion. Suspecting a possible 
parallax, he examined the observations when he had 
taken up his new office of Astronomer Royal for 
Scotland, and found a parallax amounting to 0-92". 
He did not, however, publish his results until he 
found that they were confirmed by the right ascen- 
sions. In a communication to the Royal Astronomical 
Society in December, 1838, he states that it is prob- 
able that the star has a parallax of 1-0’. 
The great and difficult problem which had occupied 
astronomers for many generations was thus solved 
for three separate stars in 1838 :— 
Modern observations 
Parallax Distance 
Parallax _ Distance 
a Centauri (Hender- , , 
SOn) secs Spat) IN) 200,000 0-750 270,000 
61 Cygni (Bessel) 0-314 640,000 0-285 700,000 
a Lyre (Struve)... 0-262 760,000 0-10 2,000,000 
(The unit of distance is that from the earth to the sun.) 
Henderson’s observation is interesting because 
a Centauri is, as far as we yet know, the nearest of 
all the stars to us. But by far the most valuable of 
these observations is Bessel’s. The heliometer, which 
he devised, proved itself to be by far the most service- 
able instrument for determining stellar parallax until 
the application of photography for this purpose. 
The somewhat dramatic manner in which the dis- 
tances of three stars were determined in the same 
year, after several centuries of failures, may have 
led to the hope that the range of many more stars would 
soon be found. This was not the case, however. 
Each star had to be measured separately, and involved 
many nights of observations. The quantities to be 
measured were so small that they taxed the resources 
of the best instruments and best observers. In 1843 
Peters published the parallaxes of half a dozen stars 
determined with the vertical circle at Pulkova, but 
the parallax of only one of these, Polaris, is obtained 
with much accuracy. With Bessel’s heliometer, 
Schliiter and Wichmann measured the distance of 
Gr. 1830, the star which had the largest known 
proper-motion. In the ’sixties, Auwers with the same 
instrument determined the parallax of several quick- 
moving stars, and also of the bright start Procyon. 
With the Bonn heliometer, Krueger in the ’sixties 
measured the distance of three stars, and Winnecke 
two more. Other observations were made, amongst 
others, by Maclear, Otto Struve, Briinnow, and Ball; 
but as these observers had not such suitable instru- 
ments, their results were not of the same high 
standard of value. A generous estimate would place 
the number of stars the distances of which had been 
satisfactorily determined before 1880 at not more than 
twenty. 
In the "eighties, progress became more rapid. Gill, 
the Astronomer Royal for the Cape, in conjunction 
with a young American astronomer, Elkin, determined 
with great accuracy, though with only a small 4-in. 
heliometer, the distance of nine stars of the southern 
hemisphere. These stars included a Centauri, and 
the bright stars Sirius and Canopus. These results 
were communicated to the Royal Astronomical Society 
in 1884. The work of Gill and Elkin did not stop 
there. After some years, a very fine 7-in. heliometer 
was obtained at the Cape, and with it, between 1888 
and 1898, the parallaxes of seventeen stars were deter- 
NO. 2379, VOL. 95] 
NATURE 

[JUNE 3, 1915 
mined by Gill and his assistants with very great 
accuracy. The stars observed at the Cape consisted 
of the brightest stars of the southern hemisphere, and 

of the stars with the greatest proper-motions. The 
results were remarkable. The stars with large 
proper-motions were nearly always comparatively near 
—say within one million times the sun’s distance. 
On the other hand, some of the very brightest stars, 
particularly Canopus, the brightest star in the sky 
after Sirius, were at vastly greater distances. 
Meanwhile, Elkin, who had been appointed director 
of the Yale Observatory in 1884, carried out with a 
6-in. heliometer, between the years 1885 and 1892, 
a determination of the distances of the ten brightest 
stars of the northern hemisphere. After these were 
finished the Yale observers, Elkin, Chase, and Smith, 
embarked on the ambitious programme of the deter- 
mination of the distances of 163 stars of the northern 
hemisphere which show large proper-motion. They 
have added forty-one southern stars to these, and 
thirty-five stars of special interest. The results of all 
these observations were published in r912. They 
have not, in most cases, the high accuracy of the 
Cape observations, but, nevertheless, are of great 
accuracy, and appear to be free from any consider- 
able systematic error. A third important series of 
observations was made by Peter with a 6-in. helio- 
meter at Leipzig. These were commenced about 1890, 
and continued until the death of Prof. Peter in tort. 
The parallaxes of twenty stars were determined with 
the same high accuracy as the Cape observations. 
Observations with the heliometer require both 
skill and industry. To secure the needful accu- 
racy measures must be made in four different 
positions of the instrument, so that possible 
small systematic errors may be eliminated by 
reversal. Great care is required in the adjust- 
ments of the instrument, particularly in the accurate 
determination of the scale-value at different tempera- 
tures. The possibility of obtaining satisfactory results 
with less labour was considered by Kapteyn, in view 
of the successful determination of the parallax of 
Gr. 34 by Auwers. From 1885 to 1887 he made ob- 
servations with the transit-circle at Leyden of fifteen 
stars for the purposes of determining parallax. The 
observation consisted in observing the time when the 
star the parallax of which was sought and two or 
three neighbouring stars crossed the meridian. Ob- 
servations are made at the two most favourable epochs 
—say every night in March, and every night in Sep- 
tember—to determine whether the star has changed 
its position relatively to its neighbours in the interval. 
The difficulties are twofold. The purely accidental 
error of observations of transits is considerable as 
compared with the small quantity which is sought. 
Besides this, the star of which the parallax is required 
is probably brighter than the comparison stars, and 
special precautions are required to guard against 
personal errors of the observer. 
In questions of this kind the only satisfactory way 
is to judge by the results. From observations made 
on fifty nights, values of the parallax are obtained 
not nearly so accurate as the best heliometer observa- 
tions, but still of considerable accuracy. Finally, the 
parallaxes of four of the stars which had been pre- 
viously determined by measures with a heliometer 
showed satisfactory agreement. 
This method has been employed by Jést at Heidel- 
berg, very extensively by Flint at the Washburn Ob- 
servatory of the University of Wisconsin, and is now 
being tried at the Cape by Vouté, a pupil of Kapteyn’s. 
It appears to me that this method can never give 
results of the highest accuracy, but that it may be 
of use in a preliminary search for stars. of large 
parallax. The argument of the facility of the method 
