296 
grant would be required to discuss the mechanism of 
respiration in different insects. I do not recollect that 
any notable progress has been made with the inquiry 
during the last thirty years, and a new observer would 
find that much remains to be discovered; it is, how- 
ever, indispensable that he should employ precise 
methods of investigation. OS Sele 
THE NUMBER AND LIGHT OF THE STARS. 
pee number of stars visible to the naked eye 
ona clear night, in the whole sky, is roughly 
5,000—a very moderate total indeed, in spite of 
the universal custom of using the number of the 
stars, in common with that of the sands of the 
sea, aS synonymous with infinity. In all ages 
mankind in general has rightly preferred rather 
to admire and wonder at the stars than to count 
them. In the great problem of the structure of 
the sidereal universe, however, which astronomers 
are now attacking with much energy and success, 
one of the essential data is the number of stars 
in the different regions of the sky, classified ac- 
cording to their brightness or magnitude. ; 
The telescope early revealed the fact that the 
stars which by reason of their superior brightness 
force themselves upon our unaided vision shine 
forth beyond a host of fainter luminaries, the 
number of which has never yet failed to show an 
increase aS an augmentation of telescopic or 
photographic power has enabled us to pierce 
depths of greater and greater obscurity. These 
fainter stars enormously outnumber the naked- 
eye stars, which may be compared with an 
oceanic island, the tiny, outstanding peak of a 
great mountain growing ever broader beneath 
the water level. 
The bright stars have been divided into six 
traditional classes or magnitudes, the brightest 
being called first magnitude stars, and the faintest 
visible being termed of the sixth magnitude. 
Pogson placed this classification on a scientific 
basis, so that it could be extended to telescopic 
stars of all degrees of faintness. On the con- 
ventional standard scale the ratio of the intensities 
of two stars differing in magnitude by the amount 
m is 10-%4™, The determination of the magni- 
tude of a star on this scale is not an easy matter, 
as it involves the measurement of the relative 
brightness of stars often differing much in 
luminosity. 
Photographic methods have been found most 
suitable for the purpose; the results are rather 
different from those obtained by visual methods, 
especially for stars of different colour, since 
the photographic plate is more sensitive than 
the eye to blue rays, and less sensitive to 
red. Magnitudes determined by ordinary photo- 
graphic methods are called “photographic mag- 
nitudes.”’ ; . 
During the past three years the fine series of 
star charts obtained by the late J. Franklin-Adams 
has been used in the investigation of the number 
of stars of determined photographic magnitudes 
in all parts of the sky. Franklin-Adams, using 
a specially-designed Taylor-Cooke to-in. lens 
NO. 2325 SVOU8g3}| 
NATURE 
| 
[May 21, 1914 
covering a wide field, photographed the whole sky 
on 206 plates each 16 in. square, the scale being 
20 mm. to 1°; the photographs were taken (the 
northern set in England, the southern in South 
Africa) with exposures of from two to two and a 
half hours, which sufliced to show stars down to 
the seventeenth magnitude on most of the plates. 
From a star of this magnitude we receive only 
one-millionth as much light as from a second mag- 
nitude star. On each plate the stars in twenty- 
five uniformly distributed sample areas have been 
counted and classified according to the size and 
greyness of their images; altogether, therefore, 
sample counts have been made on more than 5000 
regions of the sky, at intervals of 3° apart. The 
areas examined were of different sizes, being 
chosen, in accordance with the star density in the 
particular region, so that about sixty stars should 
be counted ineach. The stars could not be classi- 
fied according to their photographic magnitude 
directly from the plates, as stars of different mag- 
nitudes might show images, identical in size and 
greyness, on different plates, owing to inequality 
of the atmospheric transparency during the ex- 
posures on the two plates. The photographic 
magnitudes of a selection of the stars counted on 
each plate needed, therefore, to be determined 
directly, which was done by comparing them with 
the stars of the North Polar Sequence, a set of 
stars the magnitudes of which have been given 
very accurately by Prof. E. C. Pickering of Har- 
vard. Auxiliary photographs for this purpose 
were taken with the 30-in. reflector at Greenwich. 
After the application of corrections depending on 
their position on the Franklin-Adams plates it was 
then possible to classify all the stars counted, 
upon a true magnitude basis. This complete re- 
duction has so far been effected for thirty out of 
the 206 plates counted; the results from the 750 
areas for which sample counts were thus afforded 
have recently been published.1 
The first point of importance which will be 
mentioned as one on which the evidence derived 
from this work is decisive is that of the relation 
between the condensation of the stars towards the 
galaxy, and their magnitude. While the very 
brightest stars show little regularity of distribu- 
tion (to their irregular grouping, indeed, much of 
the beauty of the constellations is due), the fainter 
naked-eye stars show a distinct concentration 
towards the plane of the Milky Way, their density 
in the galactic belt of the celestial sphere being 
about twice that near the poles of the Milky Way ; 
still fainter stars down to the ninth magnitude 
show this phenomenon in a more marked degree, 
the density of these stars in the galaxy being three 
or four times that at the galactic pole. The 
galactic plane is a fundamental one in modern 
representations of the sidereal universe, which, 
following Sir W. Herschel’s ideas, picture the 
stellar system as formed of a large central cluster 
1 Chapman and Melotte: The number of stars of each photographic 
magnitude down to 17‘om., in different galactic latitudes. Memoirs of the 
RGA Sen ixe ads 
Chapman: On the total light of the stars. 
Monthly Notices of the 
R.A.S., Ixxiv. 
a, 
