The Washington times., 1901-10-13

! measurement of 2 02
OF the better known stars no less
| than six are of the magnitude zero
| Capella, with a measurement of 0.1S;
| Rigel, 032; Procyon, 0.46; Arcturus,
| O.G3; Vega, 019 and Sirius, minus LAB.
! Other we'll known stars with their
measurements are Algol, 2.31; Aide
| baran, 1-2O; Betelgueze, O91; Regulus,
1.i2: Affair, O.95; Deneb, L47; Antares,
LOG, and Spica, 12S.
There are comparatively few stars OF
the first four magnitudes; only 479, ac-
cording to an estimate by Prof. Simon
Newcomb. 1t would take the light of
10l6 stars from the fifth to the sixth
| magnitude to equal 12.7 stars of the
first magnitude. When it is remember
ed that the sixth magnitude star is the
smallest that can be seen with the nak-
ed eye one can better appreciate the
magnitude of the sun, especially when
one learns that all the stars thus visi-
bic are under 5000 in number and that
1t would take no less than 1900, ,OO0, ,OOO,-
600 stars of the sixth magnitude to give
us the equivalent of sunlight. Yet the
sun itself, if seen from Alpha Centau-
ri, probably the nearest of the fixed
stars of the seventh magnitude become
visible, and with a telescope with an
inch aperture stars of the ninth mag
nitude come into the field. With four
inch telescope the twelfth magnitude
may be noted, and with a 10-inch tele-
scope, the fourteenth. Theoretically, II
would require a telescope with an aper-
ture Of 6S.10 inches to discern a star of
the 18th magnitude, cut on account of
the incre9 sed thickness necessary in
lenses of the larger telescopes they new
er quite equal their theoretical capac-
ity, as compared with the smaller in
struments. On the other hand stars of
the Goth and even fainter magnitudes
have been discerned by means of pho
tographic telescopes, he great Bruce
24-inch doublet now in use at Arequipa
having enormously increased the num-
Der of stars recorded, but too faint to
be made out through the telescope.
The meridian photometer with which
the observations for the Harvard pho
tometry were made is still in nightly
use in Cambridge. over 100000 OBJ
servations have been made with it up
to the present time. 1t is composed of
two telescopes 50 arranged that the
star which it may be desired to meas
ure and another star which is taken as
the standard are thrown into the same
field by common eye-piece. By means
of mirrOr in front Of the object glass
one of the telescopes is set upon the
Pole star-the Alpha Ursae Minoris of
astronomers-which is used as the
standard not only because it is always
visible, but also because it always has
nearly the same altitude, its motion
being practically imperceptible. An /
other mirror is adjusted to bring into
view the star to be observed, which |
may thus be easily compared with the |
Pole star Since such a comparison, |
however, would be little better than the
method employed by Hipparchus or /
Ptolemy, a system of prisms is used by |
means Of which four pencils Of light, |
two from each star, are obtained. Two |
of these are then eliminated and one
rom each star reduced to the same
size. after which, for the purpose of
the first result, the process is
'eversed by using the other two pencils
IF light. When this point is reached
he observer notifies an assistant, who
s able to read the result in figures by
means Of a graduated circle, and thus
he light of the star under observation
5 expressed in terms of the Pole. star.
In all photometry the light is thus
ut off to afford an equal basis of COM
Different forms of prisms are
lsed as the case requires; and some
lines what is called a wedge" as by
Prof Pritchard of oxford and by Prof
the director of the Harvard
observatory. In the 12-inch horizontal
elescope which Prof Pickering is now
Ising the observer sits enshrouded in
cylindrical hood of felt. In the tele- {
cope is a vertically moving plate of |
lass on which the wedge or streak has I
cen photographed. grading from clear
lass to opaque blackness. The glass is
hen moved up or down till a place in ]
he wedge is reached that just cuts of
he light of the star, the assistant read I
az and recording the figures registered |
II a scale outside OF the telescope ,
-hich shows exactly how far the wedge
rithin has moved. To avoid the neces-
ity of two telescopes a little electric ;
ght is used as a standard for com
arison; but in order that no variations
4 the brightness of the light may af- 5
ect the results the mirror at the aper-
IRE is from time to time set upon some 4
tar whose magnitude has already been !
so that by means of later /
it is possible TO get the 9
lagnitUde of the stars under observa-
on irrespective of the condition of the Il
rtificial light since the standard stars !
ill vary from the accepted scale in the I
anne degree as the stars being OBJ {
erved, and the error which is allowed
0r in one case may be reckoned in an 7
ther.
The amount Of labor required in any S
xtended photometric work is remark d
BILE. At least four observations are =
lade of each star on three nights and t
many cases more. The total number T
{ stars in the northern skies which Il
ave been measured by the 12-inch mer- 6
lian photometer at Harvard is 9,233
n 40 nights Prof. Pickering personal g
made 1924 observations, or set
ngs," with this one instrument, and
n 80 nights following July q of last 3
ear no less than 1854 settings were 5
btained on 29 clear nights. Since it I
ikes at least a minute to each set of q q
jur settings some idea of the sheer L
mount of time and labor thus involved y