END-ON VIEWS OF GAS SPECTRA UNDER HIGH DISPERSION. 159 
this line, supposing its wave-number and that of the primary red nitrogen line to be very 
exactly fixed, is 1150 units, or two intervals of 575 inch-units each, which is about the length 
that the course of the intervals in the rest of the spectrum up to this unexplored portion, 
would lead us to expect. It may therefore be conjectured that the “air-line” noted in this 
place is a nitrogen-line, like the primary extreme-red one, of considerable brightness. The 
first object in the “air-spectrum” noted after it is a strong haze band occupying the place of 
the grand nitrogen leader, followed by two weaker haze bands in the places of the two 
next nitrogen groups; and finally in the extreme-red portion of the “ air-spectrum” as seen and 
mapped with end-on vision by Professor P1AzzI SMYTH in the above mentioned Table, there are, 
from the first visible one, to the red hydrogen line, ten nitrogen haze-bands or serrations very 
well recorded in appearance and position, none of the places of at least half of which had ever 
been made sufliciently visible before for measurement, so as to afford useful data for instru- 
mental determinations and for theoretical discussions. 
The special capabilities of end-on illumination, for bringing under notice and exact 
observation an immense number of details not before investigable or described, were 
exceedingly well displayed in this example. Data of the richest value, it cannot be doubted, 
are now being gathered, and views of the greatest insight and originality are in a fair way to 
be formed and fostered by the application of end-on vision and high optical perfection and 
dispersion to gas-spectroscopy ; but to have beheld the field of observation, and to have assisted 
the process as an admiring looker-on, has impressed me at the same time with the formidable 
as well as with some of the beautiful and splendid features of the scene! It may be hoped 
that photography, in its now greatly improved practice both as regards general sensibility, 
and especially in that sensibility which relates to the red end of the spectrum, will ere long 
come to the spectroscopist’s assistance, and relieve him of much the most serious and hindering 
portion of his labours, of disentangling and recording correctly what he sees. 
Now that it is well established that for a few lines sought to be produced and studied in 
a vacuum tube, a train of foreign lines and bands of contaminating gases commonly muster in 
the field, and blurr and confuse the natural spectrum sought to be examined almost beyond 
recognition, and when we further reflect that the intended spectra, if they are obtained in 
sufficient strength and purity to endure very great dispersion, are found to be of such exceeding 
intricacy as even then to surpass the means of accurate description, it will readily be admitted 
that photography would supply the spectroscopist most effectually with records of many little 
particulars of spectra containing important elucidations for his purpose, which pressure on his 
space and time constrains him, whatever art and skill he bestows upon their registration, 
to pass over unrecorded. Some sources of the irregularities of the nitrogen-spectrum-places 
were thus, it was thought, recognised in these observations, depending apparently on carbon 
and hydrogen contaminations; but they are too long and various in their nature to be here 
narrated. 
,. A common spectacle, however, both in the cyanogen and nitric-oxide tubes was a duplica- 
tion of the linelet 0’ (noted by the mean of the line-pair’s places in the Table), and occurrences 
of extra linelets, sometimes between a and 0, but more frequently on the fading-off declivity 
beyond ¢ of the second down-slope of the double-notch. Although no doubt instructive, 
discussions of these diminutive characters of the spectrum, denoting perhaps only deficiencies 
of its strength and purity, would lead to very long and laborious descriptions. <A list of 
standard line places, including three tube-carbon band-edges (red, orange, and citron) are there- 
fore added to the Table, to assist in distinguishing the groups, and to supply, in case of more 
