May 3, 1912) 
A Comparison of Dr. Peters’s Celestial Charts 
with the Photographic Charts of the Sky: J. G. 
PORTER. 
The value of star-charts lies, first, in the com- 
pleteness with which they represent the sky, and 
secondly, in their availability for use at the tele- 
scope. 
Four of the photographic charts of the sky, 
taken at Algiers and Bordeaux, were compared 
with Dr. Peters’s celestial charts, the stars in cor- 
Tesponding regions being carefully counted. In 
every case the photographic charts contain fewer 
stars, the percentages running from 51 to 81, and 
the average being 67 per cent. That is, the visual 
charts contain on the average 50 per cent. more 
stars than the photographie charts. 
Dr. Peters’s charts are pretty complete down to 
the twelfth magnitude. The photographie charts, 
therefore, are by no means complete to the twelfth 
magnitude. 
On the photographic charts the images of all 
but the brighter stars are too faint to see without 
brilliant illumination, and the configurations of 
the fainter stars are difficult to trace. Hence these 
charts are ill suited for use with the telescope. 
Two conclusions follow from this comparison. 
First, the visual charts, so far from being super- 
seded by the photographic charts, are much su- 
perior both in their fullness and in their practical 
usefulness. 
Secondly, the photographic charts while osten- 
sibly showing stars to the fourteenth magnitude, 
really go hardly lower than the eleventh and a 
half. Some of this discrepancy may be due to the 
difference between the photographic and visual 
scales; yet in any case it is clear that in the mat- 
ter of these charts photography has accomplished 
far less than was claimed for it, and less than 
should have been done to justify the expenditure 
of time and money. 
The New Twin Photographical Telescopes of the 
U. S. Naval Observatory: GEORGE H. PETERS. 
This paper is a continuation of one given at the 
meeting of the society at the Yerkes Observatory 
in 1909, entitled, ‘‘On the Construction of Astro- 
nomical Photographic Objectives at the Naval Ob- 
servatory.’’? It describes the progress in con- 
struction and adjustment of the triple photo- 
graphie objectives of 10 inches aperture and 110 
inches focal length. These new lenses and their 
mechanical parts, forming a twin photographic 
telescope, are now practically completed, and are 
SCIENCE 
105 
erected in position on the old 26-inch mounting. 
The tests for errors of adjustment in collimation 
and refraction are exemplified, together with the 
methods employed in correcting them. 
The Use of Special Topics in Teaching Astron- 
omy: SARAH F', WHITING. 
The large numbers who should study elementary 
astronomy for information and culture should be 
taught to handle the books of an astronomical 
library and to express themselves clearly in con- 
nected discourse. 
To these ends a method more frequently used 
in literary subjects may well be used—the method 
of ‘‘special topics.’? A large class for this exer- 
cise must be divided into parallel sections; the 
topics must be given out with bibliography and 
suggested outline, and the presentation of the 
topies before the class rated for excellence of 
outline, form of presentation in language and 
manner. 
Such series of topics as the following have been 
found practical. 
1. Historical topics, some of which may be pre- 
sented in connection with the biographies of as- 
tronomers. 
2. A series of topics to show the knowledge of 
astronomy at different epochs—astronomy of the 
Bible, astronomy of Homer, astronomy of Milton, 
astronomy of Shakespeare. 
3. The progress of astronomy as related to in- 
struments and mathematical methods. The devel- 
opment of the telescope, of calculus, logarithms, 
ete. 
4. A set of topics to show the immense cost in 
money and labor to obtain facts—eclipse expedi- 
tions, expeditions to obtain solar parallax. 
5. Great observatories. 
6. Special studies of celestial objects—Halley’s 
comet, net in Orion, ete. 
The Orbit of the Spectroscopic Binary, B Scorpiwi: 
J. C. DUNCAN. 
The variability of the radial velocity of the 
brighter visible component of 8 Scorpii was dis- 
covered by Dr. V. M. Slipher at the Lowell Ob- 
servatory in 1903. In 1908 he found that the 
star’s spectrum showed a sharp, non-shifting K 
line. The writer of the present paper has deter- 
mined the orbit of the binary from seventy-nine 
spectrograms made by Dr. Slipher in 1908, 1909 
and 1911. 
Measurement of the spectrograms was rendered 
difficult by the scarcity and diffuseness of the 
spectral lines. In addition to the K line, two 
