DR. W. HUGGINS ON THE PHOTOGRAPHIC SPECTRA OF STARS. 
677 
light of these bodies is distributed among a few lines only, it seems by no means hope¬ 
less to obtain on the very sensitive gelatine plates which may now be made, photographs 
of any lines which may exist in the violet and ultra-violet portions of their spectra. 
Another class of bodies to which the application of photography might give us 
much new knowledge are comets. The form of apparatus described would make it 
possible to obtain photographic spectra of the light from different parts of these bodies. 
We may entertain some hope from photographic spectra of obtaining information of 
the condition of things under which the increase and diminution of light occurs in 
those stars which are periodically variable. It is not improbable that modifications 
may be discovered in the photographic portion of the spectrum, even when none are 
seen by the eye. 
This same form of apparatus, with some obvious modification, would be useful in 
obtaining photographic spectra of the different portions of a sun-spot. 
The photographic method may also be of use in the determination of the relative 
motion of two stars in the line of sight. The photographs I have obtained of the 
spectra of two stars on the same plate do show a very small relative shift; but in an 
inquiry of so great delicacy some special arrangements, which I need not here describe, 
would be necessary to ensure the photographs from some causes of possible minute 
instrumental displacement. Also photographic spectra of opposite limbs of the sun 
on the same plate may give evidence of the sun’s rotation. 
§ VIII. White Stars. — Sirius, Vega (a Lyra), ctAquilce, a Virginis, « Cygni, ct Virginis. 
The photographic spectra of all these stars possess very strong characteristic features 
in common ; indeed, the differences between their spectra must be regarded as modi¬ 
fications of a typical spectrum common to the whole class. 
In our eye observations of stars of this class, Dr. W. Allen Miller and myself 
called attention to the intensely strong lines of hydrogen corresponding to C and F. 
Under favourable conditions of atmosphere we were able to see also, in stars of this 
class, very fine lines corresponding to the jirincipal lines of sodium, magnesium, and 
iron, though in some of these stars the least refrangible line only of b was seen. We 
remarked of these stars: “ It is worthy of notice that in the case of Sirius and a 
large number of white stars, at the same time that the lines of hydrogen are abnor¬ 
mally strong as compared with the solar spectrum, all the metallic lines are very faint.”"' 
The photographs present a spectrum of twelve very strong lines (of these seven were 
given in my preliminary note). Beyond these lines a strong continuous spectrum can 
be traced as far as S, but without any further indication of lines. The least refrangible 
of these lines is coincident with the line (y) of hydrogen near G. The next line in 
order of greater refrangibility agrees in position with h of the solar spectrum. The third 
* Phil. Trans., 1864, pp. 427, 428, 429. 
MDCCCLXXX, 4 S 
