August 20, 1885] 
NATURE 369 
corresponding advances in connection with the spectro- 
scope and sidereal photography. The three combined 
constitute a distinct feature in the more modern methods, 
by which we are gradually becoming better acquainted 
with the infinite remote. So soon as molecular physics 
shall have made, as is promised, a like advance, then the 
infinite minute also will be brought more distinctly within 
the human ken. 
With regard to the Harvard volume on Sidereal Pho- 
tometry, without unreservedly conceding to it all the 
accuracy to which it lays claim, it must be gratefully 
acknowledged that it provides astronomers with a con- 
sistent and valuable catalogue of stellar lustre which, in 
a complete form, had not hitherto existed. It dispenses 
with the too often unreliable and discordant estimates of 
the past, and replacesthem by scientific measures pos- 
sessing, to say the least, considerable precision. 
The two parts of the volume contain together no less 
than 512 closely-printed pages, many of them abounding 
with models of condensation, and constituting in them- 
selves a remarkable instance of sustained and successful 
scientific labour. They embrace not only the general 
history of the subject to which the volume refers, but 
they at the same time combine elaborate criticism and 
valuable comparisons of the results of preceding labour- 
ers in the same field. 
In the first part there is given a description of the 
meridian photometer, with which the measures of com- 
parative lustre of the stars are obtained. In it are most 
ingeniously combined the more valuable and least dan- 
gerous devices which are found in the instruments devised 
by Sir John Herschel, Steinheil, and Zéllner. Taken as 
a whole, the instrument may be properly regarded not 
only as ingenious but as original. Roughly speaking, it 
consists of two contiguous telescopes placed horizontally 
nearly in the meridian, each of the object-glasses being 
armed with a reflecting prism, so that the light from 
Polaris and any other star may be brought into the same 
field of view, after having passed through a double-image 
prism. The images are then viewed through a Nicol 
prism, and, by means well known to physicists, the light 
of the one star is reduced by a measurable amount until 
it is adjudged to be equal to that of the other star. 
We trust we may be pardoned if we suggest that this 
construction of the instrument may possibly be too com- 
plicated to admit of that amount of precision in the 
measures which could be desired, and which might be 
obtained by simpler means. In fact, it appears from the 
volume itself, that at the commencement of operations, it 
was necessary to abandon the results of several months’ 
work with it ; and although an improvement in the use of 
it was subsequently adopted, we think there still remain 
traces of the possibly inherent difficulty of precise adjust- 
ment. The rapidity also with which the equalisation of 
brightness of each star with that of Polaris is made, 
seems hardly consistent with the requisite precision. 
It is to be inferred from the volume itself that as many 
as forty-eight final determinations, each consisting of four 
equalisations of the light of a star with that of Polaris, 
are frequently completed within the hour, in addition to 
the consumption of time required for finding and identi- 
fying the successive stars and adjusting them in the field 
of view. But, we cannot doubt, this point has been well 
considered by the Harvard astronomers themselves. 
In the determination of the magnitude of a star, it is 
the usual practice to rest content, generally, with the 
mean of three determinations. Each determination is 
made on a different night, and consists of the mean of four 
equalisations of the lustre of the particular star compared 
with that of Polaris in the field of the photometer. We 
venture to think that the general limitation to three only 
is too restricted for the purposes of accuracy. The 
reason for this opinion is derived from the fact that on 
examining the numerous cases in which as many as 
fifteen determinations of magnitude are made onas many 
nights, it is very frequently, and in fact generally, possible 
to find three consecutive determinations which would of 
themselves, in the mean, lead to a magnitude widely 
different from that ultimately assigned. Yet these three 
consecutive sets furnish no circumstance of inter-discord- 
ance among themselves which could lead to suspicion, 
and which might, consistently with the usual practice, 
have finally settled the magnitude of the star in question. 
We regard this not as hypercriticism, but as being the 
only sufficient means at hand for the examination of 
accuracy furnished by the volume itself. 
Independently of the several catalogues containing the 
results of three years’ unremitting labour and persevering 
skill, the volume abounds with the intercomparison and 
reduction to one scale of the work achieved in a similar 
direction by many preceding astronomers. The result 
is that astronomers who are desirous of information 
on the subject of stellar brightness, will probably not be 
disappointed if they turn to the pages of the Harvard 
Photometry. Combined with a memoir by Prof. Pritchard, 
contained in vol. xlvii. of the WZemozrs of the Royal As- 
tronomical Society, it is perhaps not too much to say that 
all that is known upon the subject up to the present date 
will be found easily accessible to the student. 
Towards the conclusion of the volume Prof. Pickering 
has drawn up a very important table, which, though 
short, must have given him very considerable labour to 
compute. It contains in one summary a critical com- 
parison of the average results of all the principal cata- 
logues of stellar magnitude hitherto published. The 
Harvard Photometry is taken as the basis of the com- 
parison, and the difference between the mean or total 
results of each catalogue and that of the Harvard 
volume is given. From the inspection of Table Ixxxiii. it 
appears that, taken as a whole, the Harvard measures 
indicate in the mean a brightness of the stars compared 
greater than that indicated by the estimates in the Durch- 
musterung of ‘14 mag., brighter than the mean of the 
Uranometria Nova of Argelander by ‘10 mag. ; of Heis by 
12 mag. ; and of Houzeau by "11 mag. These differences, 
it will be observed, are all in one direction, and might 
appear to indicate that there is a generic difference 
between estimates of star magnitude by the unaided eye, 
and measures carefully made with a photometer such as 
is the meridian photometer at Harvard College, because 
all the estimates are apparently fainter than the measures. 
But this can scarcely be the true explanation, since the 
photometric measures also of Seidel, Zéllner, and Peirce 
indicate, like the eye estimates, a brightness less than 
that of the American determinations. Moreover, the 
photometric measures made by Prof. Pritchard at Oxford 
agree in the mean of the whole, very closely with the eye 
estimates in the Durchmusterung and the other cata- 
logues. But, whatever the significance of this fact may 
be, it cannot be doubted that the Harvard volume will 
ever remain a most valuable addition to our knowledge in 
an important branch of astronomical science. 
U.S. INDUSTRIAL STATISTICS* 
O all who study anxiously social science, this is a 
very promising publication ; its indirect testimony 
to the advantages of Republican institutions will be 
weightier to any reflective man than the eloquent tirades 
that are so usually bestowed upon them. It defines its 
object to be the stimulation and assistance of the wage- 
worker in his endeavour to reach a higher position. Its 
information respecting working men is all taken from 
their own contributions, a dozen pages of small print 
being filled with verbatim quotations from the replies of 
workpeople in every trade in the State, who give such 
r “Sixth Annual Report of the Bureau of Statistics of Labour and 
yndustries of New Jersey,” 1883.” Trenton: New Jersey, 1883. 
