64 Sidereal Astronomy . January, 
may be made on o 2 Eridan (orange and sky-blue), separated 
82" ; on /3 Cygni, of which we have just spoken in reference 
to the magnificence of their colours, and of which the two 
components are 34" apart ; an a of the Hounds (pale yellow 
and pale blue), the separation of which is 20 '; &c. 
Experiment is evidently of use only when the colours are 
complementary and belong to the two opposite ends of the 
speCtrum — that is to say, where one is yellow, orange, or 
red, and the other is green, blue, or violet. But if the two 
are red, or one red and the other yellow, — or if the two are 
blue, or one green and the other blue, — there is no interfering 
effect of contrast, and these colours are incontestable. 
It is unanimously affirmed (Arago, Humboldt, J. Herschel, 
&c.) that there is not a single green or blue star by itself in 
the entire heavens, and that this colour is only met with in 
the double stars. Having discovered (in my minute and 
detailed analysis) optical groups in which one of the 
stars is blue, I have discussed them with the greatest atten- 
tion, to be sure that these cannot be physical groups. My 
labours have resulted in the inevitable conclusion that 
simple blue stars do exist. 
Let us quote, for example, the beautiful couple of the 
double star z 2908 Cygni (7th, golden yellow ; 8*5th, blue) 
of complementary but real colours. Their distance apart 
has augmented from 9" to 21" since 1834, an d the little blue 
star has displaced itself in a direction contrary to the proper 
movement recognised in the large one. In reality the little 
one is relatively immovable in the sky, and the larger one 
nearer to us is passing before it. Their distance was at the 
minimum in 1795. This is only a perspective group, and 
the two stars are not related. There are, then, simple stars, 
which are blue. 
Among the stars visible to the naked eye I do not know 
of one which is green or blue. There are white, yellow, 
and red stars, but none which are of a green colour. 
/3 Lyrse has sometimes shown a pale tint ; this is the only 
case which exists, and again it is not authenticated ; for 
whilst many eyes consider it yellow, others see it white, and 
others somewhat green. The exaCt estimation of colours is 
more difficult than is imagined, especially in the case of 
stars, for their colours are in general faint and transparent. 
Among the brilliant stars Antares, a Herculis, Mira Ceti, 
(3 Pegasi, 8 Persii, Pollux, a Orionis, and Aldebaran are red : 
ArCturus, Castor, the Pole star, Capella, Procyon, j 3 in the 
Little Bear, a in the Great Bear, are yellow; Sirius, Vega, 
a Cygni, the cluster in Virgo, Rigulus, Rigel, s, Z, rj in the 
