DE. MILLEk’s EXETEE LEdTtJRil. 
341 
lysis enlarges the field of our enquiries to an extent which is 
really incalculable. Not merely can we, by looking through a 
prism into fiame in the midst of this room, ascertain that silver 
or iron, or both, are there. If I were to carry my apparatus to 
the top of Haldon Hill whilst you remained below, you would 
still be able to recognise the metal, be it what it might, which I 
was distilling in the voltaic arc. Nay, more, look into a furnace 
at any distance that you please through a prism, you may inter- 
pret the chemical changes that are occurring within its flame ; 
and the same method of observation may be extended to the 
outburst of a volcano, or beyond the limits of the earth, to the 
light of the sun, to the faint beams of the stars, and to the 
almost imperceptible haze of the nebulse studded here and there 
through the boundless fields of space. 
Do not, however, suppose that the foregoing observations 
comprise all that is needful to enable you to interpret all these 
wonders. Up to the present time I have shown you two kinds 
of spectra, viz., 1. the continuous spectrum, characteristic of 
the light of a glowing solid, or liquid, or cloud consisting of glow- 
ing solid particles (see Plate L., fig. 2), and 2. the irderruptecl 
spectrum, composed of ‘the bright lines which distinguish the 
spectra of glowing gases or transparent vapours (fig. 3). 3. 
Besides these there is a third kind of spectrum more remarkable 
than either, consisting of a luminous coloured band crossed by 
black lines^ shown in fig. 1. If an intensely luminous solid be 
viewed through a gas less intensely heated a very singular result 
is obtained. The spectrum of the gas is seen, as well as that of 
the solid behind it ; but the gaseous spectrum is reversed, that 
is to say, the lines of which it is formed, instead of being bright, 
are blacky as is shown in the lower half of fig. 9 ; while in the 
upper half the bright lines of sodium are seen occupying 
exactly the same position as that occupied by the dark lines 
Fig. 9, 
below, the lower part showing the appearance of a compound 
spectrum, formed by a luminous solid, in front of which is an 
atmosphere of sodium vapours. These effects you will see 
may be imitated experimentally, and the result may be shown 
on the screen. 
How are these black lines produced by thus adding light to 
light? Instances are not wanting in which sound added to 
