ASTRONOMY. 
quantity of light they afford us, compared with that of 
the Sun. And to this purpofe, Dr. Herfchel informs us, 
that, with a magnifying power of 6450, and by means of 
his new micrometer, lie found tfte apparent diameter of 
a, Lyras' to be 0 35 5". 
The ftars are diftinguiflied, with regard to their fitua- 
tion, into ajlerifms, or covjhllations \ which are nothing but 
aflemblages of feveral neighbouring ftars, confidered as 
condituting fome determinate figure, as of an animal, &c. 
from which it is therefore denominated. 
The ftars are alfo didinguifhed, in each condellation, 
by numbers, or by letters of the alphabet. This fort of 
diftindtion was introduced by John Bayer, in his Uranome- 
tria, 1654; where he denotes the ftars, in each conftella¬ 
tion, by the letters of the Greek alphabet, a, 8, 7, J, e, 
&c. viz. the moft remarkable (tar of each by <x, the fecond 
by f3, the third by 7, &c. and, when there are more (tars 
in a conftellation than the charadters in the Greek alpha¬ 
bet, he denotes the reft, in their order, by the Roman 
letters A, b, c, d, &c. But, as the number of the ftars, 
that have been obferved and regidered in catalogues fince 
Bayer’s time, is greatly increafed, as by Flamftead and 
others, the additional ones have been marked by the ordi¬ 
nal numbers 1, 2, 3, 4> 5 , &c. 
Befides the ftars thus diftinguiflied into magnitudes and 
condellations, there are others not reduced to either. 
Thofe not reduced into conftellations, are called unformed 
ftars; and thofe not reduced to clades or magnitudes, are 
called nebulous (tars; but fuch as only appear faintly in 
clutters, in form of little lucid fpots, nebula r, or clouds. 
Ptolemy fets down five of fuch nebula, viz. one at the 
extremity of the right hand of Perfetis, which appears 
through the telefcope, thick fet with ftars; one in the mid¬ 
dle of the crab called Prafepe, or the Manger, in which 
Galileo counted above forty ftars ; one unformed near the 
fling of the Scorpion ; another in the eye of Sagittarius, 
in which two ftars may be feen in a clear fky with the na¬ 
ked eye, and feveral more with the telefcope; and the 
fifth in the head of Orion, in which Galileo counted twen¬ 
ty-one ftars. Flamftead obferved a cloudy (tar before the 
bow of Sagittarius, which confifts of a great number of 
fmall ftars; and the ftar d above the right fhoulder of this 
conftellation is encompaffed with feveral more. Flamftead 
and CafTini alfo difcovered one between the Great and Lit¬ 
tle Deg, which is very full of ftars, that are vifible only 
by the telefcope. But the moft remarkable of all the 
cloudy ftars, is that in the middle of Orion’s fword, in 
which Huygens and Dr. Long obferved twelve ftars, fe- 
ven of which (three of them nerw known to be four, being 
very clofe together) feem to fhine through a cloud, very 
lucid near the middle, but faint and ill-defined about the 
edges. Caftini is of opinion, that the brightnefs of thefe 
proceeds from ftars fo minute, as not to be diftinguiflied 
by the belt glades: and this opinion is fully confirmed by 
the obfervations of Dr. Herfchel, whofe powerful telef- 
copes fhew thofe lucid fpecks to be compofed entirely of 
mafles of fmall (tars, and whofe difeoveries in the fidereal 
heavens have been already dated in the preceding part of 
this Treatife. . 
There are many dars which, though they appear fingle 
to the naked eye, are yet difcovered by the telefcope to be 
double, triple, &c. Of thefe, feveral have been obferved 
by Caftini, Hooke, Long, Mafkelyne, Hornfby, Pigott, 
Mayer, &c. but Dr. Herfchel has been by far the mod 
fuccefsful in obfervations of this kind ; and his fuccefs has 
been chiefly owing to the very extraordinary magnifying 
powers of t*he Newtonian 7-feet reflector which he has 
ufed, and the advantage of an excellent micrometer of 
his own conffrudlion. The powers which he has ufed, 
have been 146, 227, 278, 460, 754, 932, 1159, 1536, 2010, 
3168, and even 6430. He has found about 700 double dars, 
not more than forty-two of which had been noticed by any 
other perfon. Among thefe there are alfo fome dars that 
are treble, double-double, quadruple, double-treble, and 
multiple. His catalogue comprehends the names of the 
Vol. II. No. 79. 
409 
dars, and the number in Flamdead’s catalogue, or fuch a 
defeription of thofe that are not contained in it as will be 
found diffident to didinguifh them; alfo the comparative 
fize of the dars; their colours as they appeared to his 
view ; their didances determined in feveral different ways; 
their angle of pofition w ith regard to the parallel of de¬ 
clination ; and the dates when he fird perceived the dars 
to be double, treble, &e. Dr. Herfchel has diftributed 
the double dars into fix different clades. In the fird he 
has placed all thofe which require a very fupei ior telefcope, 
with the utrr.od clearnefs of air, and every other favour¬ 
able circumdance, to be feen at all, or well enough to judge 
of them ; and there are twenty-four of thefe. To the fe¬ 
cond clafs belong all thofe double dars that are proper for 
eftimations by the eye, and very delicate meafures by the 
micrometer; the number being thirty-eight. The third 
clafs comprehends all thofe double dars, that are between 
5" and 15" afunder; the number of them being forty-fix. 
The fourth, fifth, and fixth, clades, contain double dars 
that are from 13" to 30", and from 30" to 1', and from 1' 
to 2', or more, afunder; of which there are forty-four in 
the fourth clafs, fifty-one in the fifth clafs, and fixty-fix in 
the fixth clafs: the lad of this clafs is a Tauri, No. 87 of 
Flamdead, whofe apparent diameter, upon the meridian 
meafured with a power of 460 at a mean of two obferva¬ 
tions, is 1" 46"', and with a power of 932 at a mean of 
two obfervations 1" 11"'. We (hall here enumerate a few 
of the mod remarkable of thefe dars, from Dr. Herfchel’s 
catalogue : 
u Herculis, Flam. 64, a beautiful double dar: the two 
dars very unequal; the larged is red, and the fnialled blue 
inclining to green- 
( 5 1 Lyrae, Flam. 12, double, very unequal: the larged 
red, and fmalled dufky; not eafily to be feen with a magni¬ 
fying power of 227. 
a Geminorum, Flam. 66, double, little unequal: both 
white; with a magnifying power of 146 their diftance ap¬ 
pears equal to the diameter of the fmalled. 
£ Lyras, Flam. 4 and 5, a double-double dar: at fird 
fight it appears double at a confiderable diftance, and by 
a little attention each will appear double ; one fet are equal, 
and both w'hife; the other unequal, the larged white, and 
the fmalled inclined to red. The interval of the dars, of 
the unequal fet, is one diameter of the larged, with a pow. 
er of 227. 
y Andromedas, Flam. 57, double, very unequal : the 
larged reddifh white, and the fmalled a fine bright fky- 
blue inclining to green. A very beautiful object. 
«■ Urde Minoris, Flam. 1, double very unequal : the 
larged white, the fmalled red. 
(3 Lyras, Flam. 10, quadruple, unequal, white; but 
three of them a little inclined to red. 
a. Leonis, Flam. 32, double, very unequal: larged 
white, fmalled dufky. 
s Bootis, Flam. 36, double, very unequal: larged red¬ 
difh, fmalled blue, or rather a faint lilac ; very beautiful. 
b Draconis, Flam. 39, a very fmall double dar, very 
unequal : the larged white, fmalled inclining to red. 
A Orionis, Flam. 39, quadruple, or rather a double dar, 
and has two more at a fmall diftance : the double dar con- 
fiderably unequal; the larged white, fmalled pale rofe co¬ 
lour. 
| Librae, Flam, ultima, double-double: one fet very 
unequal, the larged a very fine white. 
f* Cygni, Flam. 78, double, confiderably unequal: the 
larged white, the fmalled bluifli. 
f/. Herculis, Flam. 86, double, very unequal: the fmall 
dar is not vifible with a power of 278, but is feen very 
well wdth one of 460; the larged is inclined to a pale red, 
fmalled dufkifh. 
a Capricorni, Flam. 5, double, very unequal: the lar- 
ged white, fmalled dufky. 
v Lyras, Flam. S, treble, very unequal: the larged white, 
fmalled both dufky. 
a. Lyras, Flam. 3, double, very unequal: the larged a 
3 M fine 
