28 
DBS. J. PLTTCKER AND J. W. HITTOEF ON THE 
PLATES II. & III. 
represent spectra of the second order, on a scale one-third larger than the scale of 
Plate I. 
In Plate II. N shows the second spectrum of nitrogen (20-23), O the spectrum of 
oxygen (63), S the second spectrum of sulphur (37, 38), Se of selenium (39). 
In Plate III. I shows the spectrum of iodine, Br of bromine, Cl of chlorine. Some 
remarks may be added here with regard to the conditions under which the spectra are 
obtained. 
Iodine was introduced into a bent spectral tube, and the tube exhausted as far as 
possible. While more recently tubes have been constructed which do not allow the 
discharge of Ruhmkokff’s large coil to pass, not even at a very short distance of the 
electrodes, the same effect will scarcely be obtained if iodine is enclosed in the tube. 
Accordingly the very first moment the phenomena described in art. 8 take place ; but soon 
after, vapour of iodine is developed, and by the heating power of the discharge we get, 
without the Leyden jar, a spectrum of mere iodine, consisting of very well-defined lines 
on a dark ground. After the interposition of the jar these lines became more brilliant, 
but remained well defined, and their number increased. Then the position and the 
intensity of the lines of the middle part were determined, while the red extremity 
was not at all developed, and the violet one most imperfectly. If the density of the 
vapour is increased by heating the tube by means of an alcohol lamp, the lines deter- 
mined are expanded, while the’ground becomes illuminated. The brilliancy so increases 
that the eye can scarcely bear it, till at last the discharge ceases to pass. While the 
middle part approaches to continuity, a certain number of delicate brilliant red lines, 
seen in the diagram, appear, and do not lose their distinctness as long as the discharge 
passes. Towards the violet extremity new lines likewise appear, hut though that extre- 
mity becomes most brilliant, we were not able to get the lines well defined. Accordingly 
the position of the expanded lines is approximately indicated by dotted lines. 
A drop of bromine was introduced into a small exhausted spectral tube. The tension 
of its vapour being too great to allow the discharge to pass, the vaporized fluid was 
expelled till the remaining vapour obtained a tension of about 6 centimetres. But 
by and by the vapour of bromine, combined with the platinum of the electrodes, was 
deposited on the interior surface of the tube, and after some time, evidently from want 
of sufficient conducting matter, the beautiful spectrum fainted almost suddenly. The 
spectrum was taken with the interposed jar. In this case Ha and H/3 are simulta- 
neously seen, but expanded, indicating traces of remaining water. The lines of oxygen 
are not seen. Without the jar hydrogen is not indicated. Then four bright lines, 
belonging to bromine, appear in the neighbourhood of Ha. While, with the interposed 
jar, they are fully expanded like this hydrogen-line, a less refracted subtle line appears, 
always remaining most distinct. The blue and violet extremity of the spectrum is better 
defined than in the case of iodine. 
