THE SPECTEA OE SOME OE THE FIXED STAES. 
431 
thesis of the origin of the colours of the light of the stars is also applicable to the 
remarkable exceptional class of stars the light of which is of a decided green, blue, or 
violet colour. Such stars are usually very small, and they are always so closely approxi- 
mated to other more brilliant stars, that it is scarcely possible, with the apparatus which 
we employ, to obtain separate images of the two spectra : and even were such separation 
easily practicable, the light of the strongly coloured star is usually so feeble that its 
satisfactory prismatic analysis would be a matter of great difficulty. 
[One of the objects proposed in the construction of the spectrum apparatus with 
which the additional observations on Jupiter, Saturn, and Mars were made, and which 
has been described (p. 421) in connexion with those observations, was to make it avail- 
able for the prismatic observation of some double and multiple stars. 
Before commencing the observation of the spectra of the components of a double star, 
it is necessary that the position-angle of the stars should be approximatively known. 
The spectrum apparatus has then to be rotated upon the end of the telescope until the 
direction of the slit becomes perpendicular to a line joining the stars. When the instru- 
ment is in this position, the images of the stars are elongated by the cylindrical lens 
into two short lines of light parallel with the slit, and separated from each other by a 
small interval. If the telescope be now moved in a direction at right angles to that of 
the slit, either of the elongated stellar images can, at pleasure, be made to fall upon the 
slit and form its spectrum in the instrument. By adopting this method of observation, 
the spectra of the components of f3 Cygni were separately examined. These spectra, 
especially that of B, are so faint that the lines are seen with difficulty, and scarcely 
admit of being measured. Since, however, on account of the strongly contrasted colours 
of these stars, considerable interest attaches to a comparative examination of their 
spectra, we have represented in fig. 4, Plate X., the appearances which these spectra 
present to the eye, though we have not yet measured the lines and bands in them. 
These figures must be regarded as eye-estimations only of the general features of the 
two spectra. The spectra contain, doubtless, many other lines ; and the positions of the 
lines inserted in the drawings, with the exception of b and D, were not measured, but 
only roughly estimated. The distinctive characteristics of these spectra are in accord- 
ance with the theory of the origin of the colours of the stars proposed in the foregoing 
paragraphs. In the case of both stars, the portions of the spectrum which correspond 
to the colours which are deficient in the light of the star, are those which are most 
strongly shaded with bands of absorption. Thus in the spectrum of A, the light of 
which is yellow tinted with orange, the absorption is greatest in the violet and blue ; 
for the strong lines in the orange and red, since they are narrow, would diminish in a 
much smaller degree the light of these refrangibilities. The yellow and part of the 
green are free from strong lines. 
The light of the star B appears to us to be blue, though in some states of the atmo- 
sphere the star becomes greenish blue, green, and even greenish white. These changes 
are probably due to the comparatively greater absorptive action of the vapours in the 
mdccclxiv. 3 M 
