
IN PRACTICAL ASTRONOMY, SPECTROSCOPICALLY EXAMINED. 783 
for such bands of light; and was afterwards confirmed by the discovery (inde- 
pendent at least, though it may not be original) of a black line in the Oxalate 
of Chromium and Potassium spectrum. The said black-line, when first seen in 
a weak solution, was barely separated from the left-hand, or red, side of the 
red band of light. With an increase in the strength of the solution, the black- 
line was seen further in upon the band of light. With a further increase the line 
was central on the band of light. With still further increase it came towards the 
right-hand side; and with still further increase it was lost on that right-hand side. 
Relative motion therefore of the band of light and the black-line was just 
as positively proved, as with the sun rising on the eastern horizon, passing 
through the spectator’s visible breadth of the sky, and setting in the west. 
But, that it was the band of light that really moved, and not the line, was tes- 
tified by the micrometer measures ; for they showed one and the same invari- 
- able reading for the line ; but progressively changing readings for the band. 
Similar measures on three different strengths of Didymium Nitrate solution, 
showed that all the dark lines and bands in that substance’s most remarkable 
spectrum were all of them fixed ; just as fixed indeed as the Fraunhofer black- 
lines in the solar spectrum. Lines which Fraunnorer himself, with eminent 
propriety and truth called “fixed lines;” and which we now know are pro- 
duced by absorption, some in the sun’s, and some in the earth’s, atmosphere. 
To call therefore the luminous and colour-medium spectra we are now en- 
gaged upon, simply and as though distinctively “absorption spectra,” does not 
give a sufficient idea of the chief peculiarity we have thus far found in them, 
viz., the undoubtedly locomotive, and not fixed, character of their luminous 
spectral bands ; and from this very unexpected feature something further of 
interest may be educed by and by. 
SECOND RESULT, as to certain published laws. 
The next general result of our observations is singularly opposed to the 
chief asserted “laws” of the spectra presented by coloured transparent media, 
as taught in some, happily not all, of the elementary books on Spectroscopy in 
the present day. 
Thus says one of these works,* otherwise much to be commended :— 
“(1.) In all cases the ved colour of any transmitted light, is due to the defi- 
ciency of transmitted rays belonging to the due and violet end of the spectrum. 
“(2.) Cobalt blue glass absorbs all the red and orange rays and most of the 
yellow and green, but transmits the blue rays and most of the indigo and blue- 
violet rays. 
* See p. 50, par. 3, of “The Spectroscope and its Work, 1877; by Richard A. Proctor, B.A., 
Cambridge ; being one of the “ Manuals of Elementary Science,” prepared under the auspices of “ ‘The 
Committee of General Literature and Education, appointed by the Society for Promoting Christian 
Knowledge.” 
VOL. XXVIII. PART III. 9 T 
