390 Sir David Brewster on the Lines of the Solar Spectrum, . 
even if it had been otherwise, it would have added a still more 
powerful motive, while it afforded the best apology for under- 
taking a new delineation of the spectrum. 
The apparatus which I had at my command for this inves- 
tigation were two very fine rock-salt prisms, executed by my- 
self; a large hollow prism made of plates of parallel glass for 
holding fluids; a fine plate glass prism, by Fraunhofer, and 
which I owe to the kindness of Mr. Talbot; a copious supply 
of oil of cassia and oil of cinnamon, which Mr. George Swin- 
ton transmitted to me from Bengal with his usual liberality ; a 
good achromatic telescope, by Berge; and an excellent wire 
micrometer by Troughton. ‘To this apparatus Mr. Robison 
made two important additions, which he executed with his 
own hands, the one a brass stand with a variable aperture for 
admitting the incident light, and the other a stage for holding 
and‘ adjusting the prisms in front of the object glass; and I 
have recently been favoured by Sir James South with the 
use of his fine, five-feet achromatic telescope, executed by 
Dollond. 
After a little practice in the observation of the solar spec- 
trum, I discovered most of the lines, which I had in vain 
sought for, in Fraunhofer’s map, as the counterpart of those 
in the gaseous spectrum. I saw well-marked groups, of which 
he had only given one of the lines, and shaded bands, and 
well-defined lines, which his methods of observation had not 
permitted him to discover. After I had laid down all the 
principal features in the spectrum, I was able to examine the 
two classes of lines pari passu. ‘The action of the gas upon 
invisible lines in the spectrum rendered them visible by slightly 
enlarging them, and this enlargement of a ‘solar line indi- 
cated the existence of a corresponding line in the gaseous 
spectrum. 
By this double process, and by methods of observation 
which I believe have never before been used in optical re- 
searches, I have been able to execute three different maps of 
the spectrum; first, a map of the lines in the solar spectrum; 
secondly, a map of the same spectrum, exhibiting at the same 
time the action of nitrous acid gas upon solar light, previously 
deprived of a number of its definite rays; and, thirdly, a map 
showing the action of the gas upon a continuous and uninter- 
rupted spectrum of artificial white light. The general scale 
of these delineations is, four times greater than that of Fraun- 
hofer, but some portions of them are drawn on a scale ¢welve 
times greater, which became necessary from the impossibility 
of representing in narrower limits the numerous lines and 
bands which I have discovered. The length of Fraunhofer’s 
