PRINCIPLES OF SPECTRUM ANALYSIS
PRISMATIC ANALYSIS OF THE LIGHT OF INCANDESCENT VAPOURS
DISCONTINUOUS SPECTRA
SPECTRUM BANDS PROVED BY BUNSEN AND KIRCHHOFF TO BE CHARACTERISTIC
  OF THE VAPOUR
DISCOVERY OF RUBIDIUM, CAESIUM, AND THALLIUM
RELATION OF EMISSION TO ABSORPTION
THE LINES OF FRAUNHOFER
THEIR EXPLANATION BY KIRCHHOFF
SOLAR CHEMISTRY INVOLVED IN THIS EXPLANATION
FOUCAULT’S EXPERIMENT
PRINCIPLES OF ABSORPTION
ANALOGY OF SOUND AND LIGHT
EXPERIMENTAL DEMONSTRATION OF THIS ANALOGY
RECENT APPLICATIONS OF THE SPECTROSCOPE
SUMMARY AND CONCLUSION.

We have employed as our source of light in these lectures the ends of two rods of coke rendered incandescent by electricity.  Coke is particularly suitable for this purpose, because it can bear intense heat without fusion or vaporization.  It is also black, which helps the light; for, other circumstances being equal, as shown experimentally by Professor Balfour Stewart, the blacker the body the brighter will be its light when incandescent.  Still, refractory as carbon is, if we closely examined our voltaic arc, or stream of light between the carbon-points, we should find there incandescent carbon-vapour.  And if we could detach the light of this vapour from the more dazzling light of the solid points, we should find its spectrum not only less brilliant, but of a totally different character from the spectra that we have already seen.  Instead of being an unbroken succession of colours from red to violet, the carbon-vapour would yield a few bands of colour with spaces of darkness between them.

What is true of the carbon is true in a still more striking degree of the metals, the most refractory of which can be fused, boiled, and reduced to vapour by the electric current.  From the incandescent vapour the light, as a general rule, flashes in groups of rays of definite degrees of refrangibility, spaces existing between group and group, which are unfilled by rays of any kind.  But the contemplation of the facts will render this subject more intelligible than words can make it.  Within the camera is now placed a cylinder of carbon hollowed out at the top; in the hollow is placed a fragment of the metal thallium.  Down upon this we bring the upper carbon-point, and then separate the one from the other.  A stream of incandescent thallium-vapour passes between them, the magnified image of which is now seen upon the screen.  It is of a beautiful green colour.  What is the meaning of that green?  We answer the question by subjecting the light to prismatic analysis.  Sent through the prism, its spectrum is seen to consist of a single refracted band.  Light of one degree of refrangibility—­that corresponding to this particular green—­is emitted by the thallium-vapour.

We will now remove the thallium and put a bit of silver in its place.  The are of silver is not to be distinguished from that of thallium; it is not only green, but the same shade of green.  Are they then alike?  Prismatic analysis enables us to answer the question.  However impossible it is to distinguish the one colour from the other, it is equally impossible to confound the spectrum of incandescent silver-vapour with that of thallium.  In the case of silver, we have two green bands instead of one.