f oe 4 
XX. 
NOTE ON THE SPECTRA OF CALCIUM AND MAGNESIUM 
AS OBSERVED UNDER DIFFERENT CONDITIONS. 
By WALTER NOEL HARTLEY, D.Sc., F.R.S., 
Royal College of Science, Dublin. 
[ Read, May 21 ; Received for Publication, May 24; Published, Aveusr 17, 1907.] 
Calcum.—In photographing the spark spectrum of metallic 
calcium in an atmosphere of hydrogen, and also in a vacuum 
without a jar in the cireuit, it was found very difficult always to 
obtain precisely the same spectrum, with the same exposure. The 
principal features were the bands in the red, orange, and green, 
and the line A 4226-9, and alsoin one instance there appeared very 
feebly the lines at XA 3968-6 and 3933°8. Occasionally the red 
and green bands were not so strong as they should have been ; and 
the pair of lines with the above wave-lengths appeared distinctly 
with X 42269 stronger than usual. Sometimes other lines 
appeared ; and in one instance they overlaid the red and green 
bands, as if the band spectrum was being transformed into a line 
spectrum. On one photograph of the uncondensed spark between 
metal points in a tube, at a pressure of 7 mm., the only features 
presented were the representatives of the solar lines H and K, 
namely, AA 3968°6 and 3933°8, the blue line A 4226°9, also in- 
distinctly A 4300, with feeble indications of a line on each side 
of it. There were two other lines scarcely visible about wave- 
lengths 4420 and 4452, probably 4425°6 and 4455:0. Hye- 
observations were made at the commencement of the exposures 
when the red, orange, and green bands were brilliantly visible ; but 
the exposures varied from ten to twenty minutes, and in one case 
with a small coil the period was extended to one hour. In this 
last instance it was observed that no calcium spectrum was visible, 
but only the red and blue hydrogen lines, when the spark had 
passed for some time. On developing the plate there was 
practically nothing upon it. In yet another instance, after a 
SCIENT. PROC. R.D.S., VOL. XI., NO. XX. 21 
