DR. W. HUGGINS ON THE PHOTOGRAPHIC SPECTRA OE STARS. 
G75 
form a visible spectrum, the spectrum of the sky is seen in the instrument, together 
with the spectrum of Jupiter, and much exceeding it in breadth. When the period is 
so chosen that the degree of illumination of the sky is suitable in proportion to the 
intensity of the light of Jupiter, the solar lines and those due to our atmosphere are 
well seen in close contiguity with the hires in the spectrum of Jupiter, and occupying 
exactly similar relative positions. The sky-spectrum is seen under precisely similar 
conditions of altitude and of state of atmosphere. To the light of Jupiter under 
these circumstances of observation is added the light reflected from the small area 
of sky immediately between the observer and the planet. This light is, however, too 
faint in proportion to that of Jupiter to become a source of error.”'" 
Under similar circumstances, both shutters being withdrawn, spectra of the planets 
Jupiter and Venus were taken upon the broader spectrum of the sky. The solar lines 
are thus strictly comparable with those of the planetary spectra, since they were photo¬ 
graphed under the same conditions of altitude and of terrestrial atmosphere. 
When it was desired to obtain spectra of terrestrial substances for comparison, the 
spectroscope, as a whole, was drawn out of the grooves which hold it in its place at the 
end of the telescope, and was then fixed upon a kind of optical bench, on which also 
slide two lenses of quartz, and an apparatus to hold electrodes and tubes. These are 
so arranged that an image of the spark or tube is formed upon the slit. In this way 
photographs were taken, which are comparable with those of the stars, and could serve 
for the purpose of comparison when any known line was common to both spectra to 
form a fiducial point of measurement. As all the stellar photographs contain the 
line H, calcic chloride or metallic calcium was introduced into the spark, and the line 
of calcium corresponding to was used for this purpose. 
There would have been no serious difficulty in so arranging the electrodes that a 
spectrum of the induction spark should be taken immediately after the star upon the 
same plate, but in actual practice there was some inconvenience in this arrangement. 
Two spectra on the same plate were not found to be satisfactory for comparison unless 
the “ back ” containing the plate had remained in its place. If it was removed, some 
difficulty was found in replacing it with the necessary accuracy. 
§ VI. Determination of Wave Lengths. 
The map of M. Cohnu of the solar spectrum from h to Ot, together with M. 
Mascart’s determinations of the wave lengths of the lines of cadmium in the ultra- 
violet,^ were used for the reduction of the measures to wave lengths. 
* Phil. Trans., 1864, p. 422. 
f 1 Annales de l’Ecole Normale,’ 2 e serie, tom. 3, pi. 1. 
J Mascart’s “ Recherches sur la determination des longeurs d’onde,” ‘Annales de l’Ecole Normale,’ 
tom. 4, p. 1; also Cornu’s “ Determination des longeurs d’onde des radiations tres refrangibles du Mag¬ 
nesium, du Cadmium, du Zinc et de TAluminium,” ‘Archives des Sciences Physique et Naturelle,’ 
15 Juillet, 1879. 
