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designated enhanced lines by Sir Norman Lockyer, and this name has been generally adopted. The different classes of lines are thus commonly known as name, arc, and spark (or enhanced) lines, according to their relative prominence in these three sources. This classification, however, is in some respects imperfect, and more definite designations will doubtless eventually be based upon the theoretical considerations to which reference will be made later.

There are many possible variations of these experimental methods of producing spectra, but it would seem that the equivalent of one or other of the three typical sources, or of some intermediate stage, is almost invariably obtained. A more detailed temperature classi- fication of the lines has been based upon experiments with the elec- tric furnace by King, 1 but the terms flame, arc and spark lines suffice for most purposes of description.

Similar variations have also been observed in the spectra of gases when submitted to the action of discharges of varying intensity, and the different classes of lines are sometimes distinguished by analogy as arc and spark lines, although with few exceptions the arc is not actually employed. Independent justification for these names, however, is found in the fact that the arc spectra of some gases can be directly observed. The primary and secondary spec- tra of hydrogen, for example, have both been observed in the arc, 2 and some of the principal series lines of oxygen have also been observed in the spectra of metallic arcs in ordinary air. 3 The actual spectrum given by a gas depends upon its pressure as well as upon the intensity of the discharge by which it is made luminous. Gen- erally speaking, the greater the pressure of the gas, the greater will be the strength of the discharge required to produce the " spark " lines.

One important aim of modern spectroscopic research has been to search for an explanation of these phenomena, for it cannot be doubted that the causes of the variations in the spectra are intimately connected with atomic structure. In this connexion it will be instruc- tive to refer first to the spectra of known compounds. There are many compounds which can be excited to luminosity without total decomposition, 4 and it has been found that each compound gives a characteristic spectrum by which it can be identified as such. These spectra invariably consist of bands, and different sets of bands char- acterize, for example, the oxides, chlorides and fluorides of the alka- line earth elements.

It is sufficiently obvious that if a compound be stimulated so strongly that it becomes dissociated, the spectrum will change from one consisting of bands representative of the compound to one con- taining the lines of the constituent elements. It is not only com- pounds, however, that show changes of this character. Experi- ments on nitrogen, for instance, show a range of spectra from one consisting wholly of bands to one in which lines occur alone. Even hydrogen has two spectra: (i) the highly complex, so-called sec- ondary spectrum, which doubtless represents a banded spectrum of rather coarse structure 5 ; and (2) the familiar line spectrum, con- stituting the Balmer series. Similar results have been obtained for many other elements, and from analogy with the spectra of com- pounds the natural conclusion is that the band spectra of the ele- ments arise from molecules, while the line spectra are produced by the atoms which are set free when the molecules are dissociated.

If this be a true view, the change in the structure of the atom, or in its mode of vibration, which accompanies the successive modifica- tions of the line spectrum becomes a question of paramount interest. Lockyer 6 did not hesitate to believe that while the arc lines of an element were to be attributed to ordinary atoms, the enhanced lines could only be produced by the splitting-up of the atoms them- selves, and he called these simpler forms of matter the proto-ele- ments. Proto-calcium, for instance, denoted calcium which had been broken up into sub-atoms by the application of a sufficient stimulus. A somewhat similar, but more probable, explanation has been based upon an application of the quantum theory by Bohr to Rutherford's nucleus theory of the atom. This theory is founded largely on the analysis of spectra into regular series.

Range of Observations. For the complete determination of the laws of spectra it is necessary to extend the observations far beyond the limits of the visible spectrum. Conspicuous success in the direct photography of the near infra-red spectrum has been achieved by Meggers and others, by the use of ordinary plates stained with dicyanin. 7 By this method excellent photographs of the arc spectra of a large number of elements, extending to Xio.ooo, have been obtained with a concave grating, and the positions of the lines have been measured with a high degree of accuracy. For the present,

1 Several papers in the A strophys. Jour.

2 Fowler and Shaw, Proc. Roy. Soc. A Ixxxvi., 128 (1912).

' Meggers and Kiess, Sc. Pub. Washington Bur. of Standards, No. 324, p. 644 (1918). .

R. J. Strutt (now Lord Rayleigh), is particularly effective for the spectra of many compounds. Proc. Roy. Soc. Ixxxvi., 105 (1912).
 * Stimulation by " active nitrogen, according to the methods ot

6 An excellent photographic map of this spectrum has been given by T. R. Merton, Proc. Roy. Soc. A xcvi., 382 (1920).

6 Lockyer, Inorganic Evolution (1900).

7 Scientific Papers, Bureau of Standards, No. 312 (1918), and sub- sequent papers.

the extreme infra-red can only be investigated by thermal effects, involving the use of the thermopile, bolometer, or radio-micrometer, as in the researches of Paschen, Lehmann, and Randall.

Spectroscopic observations in the direction of the ultra-violet, beyond the limit about XiSso set by the absorption of quartz, 'and beyond about Xiyoo set by the absorption of air, which were first made by Victor Schumann, have been greatly extended by the use of concave gratings, and wave-lengths of considerable accuracy have been determined. Lyman has recorded lines as far as \soo angstroms, and in similar work at Toronto, McLennan has observed a line attributed to carbon at X584. A still greater extension has been made at Chicago by Millikan 8 and his colleagues, who have observed lines of nickel as far as X2O2. Several improvements in technique were necessary to this success. It was achieved, in the first place, by using gratings specially adapted for the purpose; secondly, by working in an essentially perfect vacuum, through the use of powerful pumps; and finally, since no ordinary spark could pass in a vacuum, by the use of a specially strong sparking apparatus which was capable of forcing a discharge across a very small space between the electrodes. Such a spark was found to produce the extremely short X-rays in the case of carbon, so that the gap which had previously existed between ordinary light waves and X-rays was for the first time bridged.

Spectroscopic data thus cover a very wide range, and offer many interesting problems to the investigator. Their solution depends on his ability to make a true analysis of spectra, and to deduce there- from the corresponding atomic or molecular conditions.

Analysis of Spectra. Considerable progress has been made in the analysis of spectra into series. One of the most important advances in this direction is the increased knowledge of the primary spectrum of hydrogen, which is now known to contain, not only the Balmer series, but also two similar series, one in the infra-red and the other in the far ultra-violet. The former was discovered by Paschen, and the latter, previously predicted by theory, was found by Lyman with his vacuum spectrograph. Each of these series is well represented by a mathematical formula, which is simplified if the lines are expressed by their " wave-numbers " (v) instead of their wave- lengths. The wave-number is the number of waves per centimetre in vacua, and is proportional to the frequency of the vibration. In practice, it is obtained by dividing the wave-length (in angstrom units), connected to a vacuum, into IO 8. In these terms, the for- mulae for the hydrogen series are as follows :

Lyman series: Balmer series:

Paschen series :

N N. v = * (m

2, 3,

N N ,

v = ^2^z (" I= 3i4.

'N N ,

v= ^ 2 2 (>=4i5.

Thus, a general formula for the primary hydrogen spectrum, which

N N might include other undiscovered series, would be v =.

N is a constant, whose value 9 is 109678-3. OT is a constant integer for any one series; and m z has a different integral value for each line of a series. R. W. Wood 10 has recently extended the Balmer series to m = 22, i.e. to 20 lines, by experiments with long vacuum tubes. In the spectrum of the sun's chromosphere, 34 lines of the series have been recorded.

There is only one other known spectrum which has the same sim- plicity as that of hydrogen namely, the enhanced spectrum of helium. This includes the series first found by Pickering in the star f Puppis, and the line X4686 and others calculated by Ryd- berg, by whom both series were attributed to hydrogen. These lines were produced in the laboratory by Fowler, 11 and additional lines of the Pickering series, first indicated by Bohr's theory, were afterwards observed by Evans 12 and by Paschen. 13 It was, in fact, the theoretical work of Bohr which first suggested that the lines in question originated in helium and not in hydrogen.

The enhanced series of helium can be represented by a formula similar to that for hydrogen, with the difference that the series constant has rather more than four times the value for hydrogen.

Thus, the series which includes X4686 is given by c=4N'(-j-^

when N' is 109723. The complete Pickering series is given by sub- stituting I/4 2 for I/3 2 in this formula, and a further series calculated by the use of 1/2 2 has been partially observed by Lyman. It should be noted that alternate lines of the Pickering series are nearly coin- cident with the Balmer series of hydrogen.

"Astrophys. Jour. Hi., I (1920).

9 W. E. Curtis, Proc. Roy. Soc. A xcvi., 147 (1919).

10 Proc. Roy. Soc. A xcvii., 455 (1920).

11 Monthly Notices R. A. S. Ixxiii. , 62 (1912); Phil. Trans. Accxiv.,

254 (1914)-

13 Ann..d, Phys. I, (1916)-
 * , . " Phil. Mag. xxix., 284 (1915).