The New Student's Reference Work/Spectroscopy

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Spectroscopy (spĕk-trŏs′ kṓ-py̆) is a science which has for its object the determination and description of the various radiations which bodies emit, reflect and absorb.  It is to be carefully noted that spectroscopy does not end in the observation of phenomena, but includes also the description of these phenomena in a manner which is at once the simplest and most complete.

Method of spectroscopic science[edit]

The examination of a body by means of the spectroscope includes four steps.  These are the Production of Radiation; the Separation of the different Radiations; the Recording of the Radiations; and the Comparison of the Radiations.  We shall consider the subject from these four points of view.

A.  What the spectroscope receives and analyses is radiant energy.  It follows naturally, therefore, that the first step in examining the spectrum of any body is to make that body a source of radiant energy.  In many cases this step has already been performed for us.  Thus the fixed stars and the gases in which lightning-discharges occur are already rendered self-luminous.  All we can do, therefore, is to observe their spectra; we cannot experiment upon them.  Bodies at the surface of our earth can generally be made to radiate energy (1) by heating them, as in the case of a red-hot poker; (2) by passing an electric current through them, as in the case of the Plücker tube; (3) by means of chemical combination, as in the case of the Bunsen flame; or (4) by means of luminescence, of which fluorescence and phosphorescence are examples.  As to just how and why these processes cause bodies to emit radiant energy very little is known.

Fig. 1.  A prism-spectroscope adjusted to view one particular color in the spectrum
Fig. 2. Grating-Spectroscope

B.  Passing now to the second step, namely, the separation of the radiant energy of one wave-length from that of the others, this is the peculiar function of the spectroscope, and is usually accomplished in one of two ways: either by interposing in the path of the ray a prism which impresses upon each ray of different wave length a different direction; or by placing in the path of the rays a diffraction-grating which accomplishes the same thing.  (See Prism and Diffraction-Grating.)  Of these two methods, the first was introduced by Newton about the middle of the 17th century, and the second by Fraunhofer near the beginning of the 19th century.  The ordinary prism-spectroscope, shown in figure 1, is generally made up of three parts, viz., a prism and two astronomical telescopes.  One of these telescopes is used to render the rays which fall on the prism parallel to each other, and is therefore called the collimator.  The other telescope is made movable so that it can be turned into the proper direction for observing any desired color, and is, therefore, called the view-telescope.  A grating spectroscope is essentially of the same construction (see figure 2) except that the dispersion, i. e., the separation, of the rays is produced by the grating at G, instead of by a prism.  The radiant energy is here admitted through the small narrow opening at S2, called the slit.

An image of this slit is produced in the focal plane of the view-telescope, and in the ordinary spectroscope is there examined by means of an eyepiece.  But, if we place a small camera at the end of the view telescope in such a position that the photographic plate will lie in the focal plane of the telescope, we can then obtain a photograph of these images.  Such an instrument is called a spectrograph.

Among the most powerful spectroscopes known must be mentioned the curved grating instrument devised by Rowland.  For the best account of this see Kayser’s Handbuch der Spectroscopie.  In the same class must be mentioned Michelson's echelon spectroscope, which is well-described in Drude’s Lehrbuch der Optik.

C.  Passing now to the manner in which spectra are recorded, this depends partly upon the portion of the spectrum under examination and partly upon the purpose of the examination.  If it is merely the appearance of a line, a glance of the eye may suffice, provided the line is in the visible part of the spectrum.

But in the ultraviolet part of the spectrum photography is practically the only available means for obtaining a record.  In the infrared portion of the spectrum we cannot see and cannot easily photograph what is there; so that here the radiomicrometer or bolometer or radiometer must be used.  In general, photography is the best and most easily worked method whenever it is applicable.

D.  Comparison of Spectra.  Let us suppose now that by some means we have analyzed the radiation from a certain body and have recorded its spectrum.  The next step in the process is to apply this information to the purpose for which it was obtained.  This last step — the interpretation of results — perhaps is the most important one of the four, and the one which demands, in its execution, the largest measure of care and experience.  After a spectrum is once obtained it becomes generally necessary to compare it (1) with itself when the radiation is produced under different conditions; (2) with itself when recorded in another way; (3) with the spectra of other bodies; (4) with some spectrum which has been predicted by theory; or (5) with a standard scale of wave-lengths; that is, with an ideal spectrum in which each line differs from its neighbor by one of the units in which wave-lengths are measured.

History of spectroscopy[edit]

A bare outline of the steps by which this science has reached the high degree of perfection which it to-day possesses would be something like the following:

1.  Newton, by his careful study of the prism, gave a fairly complete answer to the question of separating the various radiations.

2.  Young and Fresnel, by establishing the wave-theory of light along the lines laid down by Huygens, placed spectroscopy upon a sound scientific basis.

3.  Fraunhofer, by inventing the diffraction-grating, greatly increased the power and simplicity of the instrument.

4.  Kirchhoff and Bunsen (1859) showed that each chemical element emits a characteristic radiation and, thereby, established the science of spectrum analysis.

5.  Rowland, by the perfection of the plane grating and by the invention of the curved grating, as well as by his superb published spectra of the sun and elements, revolutionized the entire science and gave it a precision hitherto undreamed of.

6.  Kayser and Runge, by their study of the spectra of the elements, have shown that there is an order, and not a chaos, running through the thousands of lines already mapped.

7.  Michelson (1890–1900) has shown us how a single line in any spectrum may be analyzed and studied in detail; and has thus, at once, multiplied the power and the problems of the science.  This he has accomplished by the invention and use of the interferometer and the echelon spectroscope.  Kayser’s Handbuch der Spectroscopie is incomparably the best treatise ever written on this subject.

Henry Crew.