$86 Sir David Brewster on the Lines of the Solar Spectrum, 
degrees towards the red and the violet extremities. In the 
year 1808, Dr. Wollaston conceived the happy idea of examin- 
ing a beam of light, that passed through an aperture only 
the twentieth of an inch wide, and he was surprised to see it 
crossed by seven dark lines, perpendicular to its length. 
About ten or twelve years afterwards, the celebrated op- 
tician Joseph Fraunhofer, without knowing what had been 
done by Dr. Wollaston, observed the spectrum formed by the 
sun’s light transmitted through small apertures; and by ap- 
plying a telescope behind the prism, he discovered about 600 
parallel dark lines traversing the spectrum. As no such lines 
appeared in the spectra of white flames, Fraunhofer considered 
them as having their origin in the nature of the light of the 
sun. The strongest of these lines were seen in the spectra of 
the Moon, Mars, and Venus, and, by means of very fine in- 
struments, he was able to detect one or two of them with 
other new lines in the spectra of Sirius and Castor. 
Such was the state of the subject, when I made the experi- 
ment already referred to on nitrous acid gas. Upon examin- 
ing with a fine prism of rock-salt, with the largest possible 
refracting angle, (nearly 78°,) the light of a lamp transmitted 
through a small thickness of the gas, whose colour was a very 
pale straw yellow, I was surprised to observe the spectrum 
crossed with hundreds of lines or bands, far more distinct 
than those of the solar spectrum. The lines were sharpest 
and darkest in the violet and blue spaces, fainter in the green, 
and extremely faint in the yellow and red spaces. Upon in- 
creasing, however, the thickness of the gas, the lines grew 
more and more distinct in the yellow and red spaces, and be- 
came broader in the blue and violet, a general absorption ad- 
vancing from the violet extremity, while a specific absorption 
was advancing on each side of the fixed lines in the spectrum. 
It was not easy to obtain a sufficient thickness of gas to de- 
velop the lines at the red extremity, but I found that heat 
produced the same absorptive power as increase of thickness, 
and, by bringing a tube containing a thickness of half an inch 
of gas to a high temperature, I was able to render every line 
and band in the red rays distinctly visible. 
The power of heat alone to render a gas, which is almost 
colourless, as red as blood without decomposing it, is in it- 
self a most singular result; and my surprise was greatly in- 
creased when I afterwards succeeded in rendering the same 
pale nitrous acid gas so absolutely black by heat, that not a ray 
of the brightest summer’s sun was capable of penetrating it. In 
making this experiment, the tubes frequently exploded, but, 
by using a mask of mica, and thick gloves, and placing the 
