466 Proceedings of the Royal Society 
flames, and it would appear that this w T as a property which belonged 
to almost every flame.” 
One thing only was wanting in order to complete this discovery 
of Brewster’s, namely, to explain why the rays which are bright 
in artificial flames should be dark in the solar spectrum. The ex- 
planation of this fact was reserved for later inquirers. 
The above is far from exhausting the catalogue of Brewster’s 
researches on the spectrum. He made numerous measurements of 
Fraunhofer’s lines and maps of certain portions of the solar 
spectrum. He likewise discovered the extraordinary effect of 
nitrous gas upon the spectrum transmitted through it, which 
becomes covered with a vast multitude of lines, irregularly dis- 
posed, but always appearing in the same places in the spectrum, 
provided the density and temperature of the gas is the same. 
2. On Some Optical Experiments. By H. F. Talbot, Hon. 
F.R.S.E. 
I. On a New Mode of observing certain Spectra. 
The attention of the scientific world has been for some years 
past fully awakened to the importance of observing the spectra 
exhibited during the combustion of chemical substances. But in 
making an extensive series of such experiments, it must often hap- 
pen that the observer has to test substances of which he only pos- 
sesses a very minute quantity. In that case, before he has viewed 
the spectrum long enough to feel fully satisfied of its nature, his 
stock of the substance is exhausted, and he is obliged to leave his 
observation imperfect. He might perhaps he testing some mineral 
in his cabinet, of which the native locality was unknown, and he 
might surmise it to contain a new metal, from its yielding a ray 
not before seen in the spectrum, yet after a short time his observa- 
tions on it would come to an end, and he would have no means of 
showing this ray to other observers. Some years ago the metal 
thallium was so rare that it was only distributed by a few grains at 
a time to those who were interested in its discovery; and many of 
the rarer metals are absent from most chemical laboratories, or onjy 
represented by trifling specimens. About four or five years ago I 
devised a method of remedying, or, at least, greatly diminishing 
