Sidereal Astronomy. 
63 
1876.] 
The 101 stars of which the components are of the same 
colour, but of different intensities, give for the same dif- 
ference — ■ 
1*144. 
And the 120 stars of which the components are of different 
colours give the number — 
1*883. 
The disproportion is such that there can remain no doubt 
that difference of colours is so much stronger in proportion 
as the difference of the magnitudes is itself greater. The 
greatest intervals of magnitude are not between the yellow 
and blue, but between green and blue stars. 
Now are all these colours real, or will there not rather be, 
in most cases, an effedt of contrast ? A white star placed 
by the side of a very brilliant red star becomes green by 
contrast. The retina of the eye, excited by light of one 
determined colour, becomes insensible to a less intense light 
of the same colour, and only sees the complementary 
colour. 
Red produces green. 
Orange ,, blue. 
Yellow ,, violet. 
And reciprocally. 
To know if these colours are real, and not due to the 
effedt of contrast, it is necessary, when possible, to hide one 
of the stars while looking at the other ; the effedt of con- 
trast disappears with its cause, but not immediately, as is 
generally believed. It takes a certain time — many seconds, 
and sometimes nearly half a minute — for the eye not to be 
influenced. It is for that reason difficult to experiment 
when the two stars of a couple are at a small angular dis- 
tance. The diurnal movement magnified by the eye-piece 
quickly causes the star to emerge from behind the wire 
which has been placed before, even when the telescope has 
an equatorial movement. But the experiment is easy if we 
take stars which do not touch. One of the easiest of 
the examples, even for a moderate instrument, is that of the 
triple star o 2 Cygni. The large star is of the fourth magni- 
tude ; the second, of the 6*5th magnitude, is distant 1' 47"; 
and the third, of the fifth magnitude, is at a distance of 
5' 38". The diversity of the colours is surprising ; the first 
is of a fine yellow, and the other two are blue. If, then, we 
hide the largest, and look only at the two little ones, they 
entirely retain their blue colour. The same observation 
