Oct. 7, 1886 | 
NATURE 
561 
the historical data, which are of easy access to every investi- 
gator, I may mention that No. 77 of the Astronomical Fournal 
contained nineteen photographic impressions of as many different 
phases of the solar eclipse of 1854, May 26—the moment of 
each impression being given to the nearest tenth of a second. 
These were taken at West Point, under the direction of Prof. 
Bartlett, of the U.S. Military Academy, and form a part of his 
memoir, in which he also gives the distances between the cusps, 
as measured by himself with the micrometer in the telescope. 
Ten years later, in 1864, Mr. Rutherfurd constructed the 11}- 
inch photographic object-glass which has acquired so _con- 
spicuous a place in astronomical history; and with this, in 
addition to its other achievements, he obtained sharp photo- 
graphic stellar images, with a definition previously unknown, 
taking for the first time distinct impressions of stars invisible to 
the naked eye, in fact to the 8} magnitude for white stars. 
After constructing a micrometer of great delicacy for the 
measurement of these plates, he measured with this the relative 
distances and position-angles of the stars which they contained. 
And in the spring of 1866 he kindly placed in my hands the 
results thus derived from three plates of the Pleiades, each con- 
taining two impressions, taken on the evening of March Io. 
One of these plates contained forty stars. Bessel’s memoir 
upon the Pleiades, published in 1844, gave the relative positions 
of fifty-four stars, measured with the Konigsberg heliometer, 
during the years 1829 to 1841. Six of these fifty-four do not 
belong within the limits of the plate (which contains about one 
square degree), and ten of them are too faint for the photo- 
graphic record, so that sixteen of Bessel’s list are wanting ; but, 
on the other hand, there are two additional ones, not observed 
by him. 
From this fact alone it may be perceived that among the great 
benefits which astronomy may be justified in expecting from 
celestial photography, the accurate determination of magnitudes 
does not find place. The chemical ac’ion of the stellar light 
upon the film is so dependent upon the character of that light 
that, in the absence of a correct knowledge of its composition, 
we are very easily deceived regarding the amount. Thus one of 
Bessel’s stars which was not recorded upon any of Mr. Ruther- 
furd’s plates is estimated by Argelander as of the magnitude 8:0, 
and by Wolf as 7%, while five are distinctly recorded which 
Argelander calls 84 or less, and eight which Wolf so estimates. 
The spectroscope would doubtless show a deficiency of the 
more refrangible rays in the light of the former, and a pre- 
ponderance of the same in that of the latter. 
This series of measurements by Mr. Rutherfurd, together with 
the computations to which the results were submitted, constitute, 
if Iam not mistaken, the first application of the photographic 
method to exact astronomical determinations. - And the investi- 
gation necessarily demanded especial care, both for guarding the 
numerical results against sources of unsuspected error and for 
fixing the limits within which known theoretical errors would 
remain unappreciable. 
The importance of the successful application of a method so 
different from all previous ones, and so full of promise, and also 
the considerable time which would inevitably elapse before the 
memoir could be printed, led me at the same time to communi- 
cate to the Astronomische Nachrichten, at Altona, some of the 
resultant values. In a comparatively short note, written about 
the middle of August 1866, I gave for the ten most conspicuous 
stars of the Pleiades, after Alcyone, the corrections derived from 
one of the photographic plates of March Io, for the values, pub- 
lished by Bessel, for the position-angles and distances, from 
Alcyone in 1840, as likewise the average discordance found for 
a single measure. 
In the next following year the Academy had not the means of 
printing its memoirs ; and as in the meanwhile Mr. Rutherfurd 
had measured five more of the plates of the Pleiades previously 
taken, as well as six additional ones taken in the months of 
January and February 1867, these were also computed, and the 
results added to those from the first three plates in the memoir 
already written. 
Various circumstances combined to delay the publication, 
chief among them being what seemed to me a manifest im- 
propriety in printing the results derived from photographs and 
measurements made by Mr. Rutherfurd, and by his own methods, 
before some account of these methods should have been pub- 
lished by him. His communication on the subject had been 
made to the National Academy immediately previous to my 
own, but was not yet in such form as he desired for publication. 
The result showed a very remarkable accordance with Bessel’s 
determination for 1840, although the total amount of relative 
proper motion during the elapsed twenty-six years was comprised 
in the differences. 
This memoir still remains in its original form, but un- 
published ; the results being deduced from twenty-four photo- 
graphic impressions upon fourteen plates. 
In the next year, 1868, I had the gratification of receiving 
from Mr. Rutherfurd the results of his measurements of thirty- 
two stars of the cluster Prasepe, derived from eleven impres- 
sions. These were computed in the same way that those of the 
Pleiades had been, and an analogous memoir upon this cluster 
was prepared for the National Academy. 
Before leaving the country, early in 1870, I gave these two 
memoirs to Mr. Rutherfurd, with the request that he would send 
them to the printer, at the same time with his own paper, already 
mentioned, but not before then. ‘lhe condition of his health 
prevented him from attending to the matter for some time, and 
in the interval he arrived at the unpleasant discovery that the 
screw of his micrometer had suffered from wear, and to an 
extent which led him to fear a want of that accuracy of which 
the method is susceptible, and which he hoped to see demon- 
strated by its very first applications. 
Notwithstanding this possible blemish, it seems to me that 
the results ought to be now made public in their original form, 
after due mention of the circumstances ; and it is among my 
hopes to be able soon to publish these two memoirs from the 
original manuscript of so many years ago. 
The method was received with manifest distrust and disregard 
abroad ; and, as was but natural for so essential a deviation 
from former methods, very many grounds of criticism and objec- 
tion were brought up. One of the principal of these was the 
possible distortion of the collodion film, after receiving the im- 
pressions and before the measurements; but Mr. Rutherfurd 
speedily disposed of this point, at least so far as the albumenised 
plates are concerned ; and, moreover, the combination of mea- 
surements of the same stars derived from various plates will at 
once make manifest the degree of confi ence to which the 
several values and their wear are respectively entitled. 
A far more serious obstacle to accuracy is presented by the 
difficulty of obtain‘ng absolutely round images. Ivregularity of 
form in the dots formed by the stellar impressions is almost in- 
compatible with precision of measurement ; and, as the time of 
exposure must often be long, the chief problem was, not so much 
to obtain the images as to insure uniformity of motion in the 
telescope during the period of exposure. Not that the photo- 
graphic processes were not troublesome enough before the intro- 
duction of the dry-plate processes, for very great care and numer- 
ous precautions were often necessary to prevent the plates from 
drying too fast: but far the greatest difficulty consisted in obtain- 
ing sufficient precision in the clockwork and equatorial motion of 
the telescope. 
It may easily be imagined how great was my desire, when 
leaving home for South America, to extend this new method of 
observation to the southern hemisphere. But the obstacles 
encountered in the endeavour cannot be easily imagined. Upon 
these I will not enlarge here further than by saying that in 
Cordova also the attainment of circular dots for the star images 
offered incomparably the greatest of all the difficulties of a 
practical character. The time of exposure was limited by the 
maximum size allowable for the large stars, and, previous to 
1878, also by the drying of the plate, although exposures of 
twenty minutes were not unusual. Nevertheless, by dint of 
specially constructed governors and regulators, and by ceaseless 
attention, we did succeed in obtaining impressions which, to the 
unaided eye, appear absolutely round. 
This necessity of long-continued and minute uniformity in the 
motion of the telescope is, of course, largely diminished by the 
employment of instruments of large aperture, inasmuch as the 
necessary time of exposure is diminished in the same ratio in 
which the amount of light is increased. It is yet further and 
most notably diminished by the manifold greater sensitiveness of 
the dry gelatine plates. But notwithstanding all this, the attain- 
ment of round images, while almost indispensable for giving to 
stellar photography that increased accuracy to which it may lay 
claim as a means of research in practical astronomy, still 
demands especial care and precaution. 
The Argentine Government cordially afforded every assist- 
ance which I deemed it proper to ask for these investigations. 
And although the chief energies of the Cordova Observatory 
