1864. | Srewart on Radiant Light and Heat. 593 
has drawn it as the two strong lines with a Fic. 4. 
faint one between, shown in our cut. Pro- 
fessor Cooke, of America, has still further 
filled up the intervening space, while very 
lately Mr. Gassiot, by his magnificent spectro- 
scope of eleven sulphuret of carbon prisms, 
has’ obtained the appearance presented in 
Fig. 4. 
In order to give our readers an idea of the amount of separation 
between the different kinds of light produced by the prism, we may 
mention that Kirchhoff lengthens out the visible spectrum into 8 feet, 
while a map is being prepared at Kew Observatory from Mr. Gassiot’s 
spectroscope, which, when finished, will probably attain the very great 
length of 24 feet. But even with instruments of the highest power, 
we yet perceive nebulous bands in the spectrum, which a still higher 
power might not improbably resolve into lines; just as with Lord 
Rosse’s telescope we observe nebule in the heavens which a higher 
power might possibly resolve into stars. 
Spectra of the fixed stars have likewise been obtained by several 
observers. Fraunhofer noticed that the double line p was present 
in some of these. Secchi, Airy, and others, and more lately Huggins 
and Miller, have obtained the spectra of several stars. The last- 
mentioned observers, in particular, have furnished detailed maps of 
those of Aldebaran, Alpha Orionis, and Beta Pegasi. We shall 
return again to this subject, but in the meantime we may remark that 
the general character of stellar spectra is a luminous ground inter- 
sected by dark lines, many of which are identical with lines in the 
solar spectrum, while others however are different. 
But we must not omit to mention the appearance presented by, the 
spectra of heated vapours. Electricity and other agents enable us to 
obtain the vapours of metals and other bodies at an extremely high 
temperature, and when the rays which these emit are analyzed by the 
prism in the way already mentioned, we have results which are exceed- 
ingly curious. The metal sodium, or any of its compounds, such as 
common salt, produces a flame which is intensely yellow; and this 
flame, when analyzed by the prism, is found to consist of two simple 
rays exactly corresponding in position with the two lines p. We are 
thus furnished in the salt flame with two rays which are absent in the 
light of the sun, and with these rays alone, for there is not a vestige 
of any other, and we have a perfectly dark ground—the appearance 
being that of the upper portion of Fig. 5. Again, when the vapour 
which we examine is that of iron, we have as before bright lines on a 
dark ground, and these have been found by Kirchhoff to coincide with 
certain dark lines in the solar spectrum. Generally speaking, all 
heated vapours give spectra, consisting of bright lines scattered over a 
dark space, but in many of these the bright lines have not as yet been 
found to coincide with dark solar lines. It has been shown by Bunsen, 
Kirchhoff, and others, that these bright lines are always character- 
istic of the vapour which is ignited to produce them. Each ele- 
