INTRODUCTION. 6 



to pervade not only the space between atoms but also the space within 

 atoms and electrons; and is also assumed to be at rest. There will be an 

 electromagnetic field within the electron as well as without. Various dis- 

 tributions of charge may be assumed. Lorentz usually assumes a volume 

 density (,o) distribution such that p is a continuous function of the coordi- 

 nates. The charged particle has then no sharp boundary, but is surrounded 

 by a thin layer in which the density gradually sinks from p to 0. The ether 

 is simply the space in which a certain state of the electromagnetic field 

 exists. (Recently very interesting papers by Einstein and others on this 

 subject have appeared.) The electron having been thus defined, equations 

 can be formed for the electric and magnetic fields for any region in which 

 there are electrons either at rest or in motion. 



Having considered the elementary unit of the mechanism of optical 

 phenomena, let us now turn to some of the phenomena themselves. 



(a) SPECTRA OF GASES. 



For optical purposes bodies may be divided into gases and into solids 

 and liquids. The spectra of gases consist of an enormous number of fine 

 lines and are usually grouped into line and band spectra. Band-spectra 

 themselves consist of a great number of sharp lines spaced in a very regular 

 manner, whereas line-spectra consist of lines apparently spaced more or 

 less at random in the spectrum, although some of the lines have been found 

 to have frequencies that are connected by certain series relations. Good 

 examples of band-spectra are the absorption spectra of fluorine, bromine, 

 iodine, chlorine, sulphur, or sodium vapors. These consist of thousands 

 of very fine lines. Very interesting work has recently been done by Wood 

 on the magnetic-rotation spectrum and the fluorescent spectrum of sodium 

 vapor. On exciting fluorescence by monochromatic light of different wave- 

 lengths it is possible to set into vibration apparently different systems in 

 the sodium atoms or clusters containing sodium atoms, each one of these 

 systems of vibrators emitting a different series of bands. It is found that 

 the presence of foreign gases has a very great effect upon the absorption 

 spectra of sodium. The presence of hydrogen prevents fluorescence. 

 Wood * found that as mercury-vapor is evolved in a vacuum the band 

 X 2536 broadens rapidly on the less refrangible side, attaining a width of 300 

 or 400 Angstrom units. There is a little broadening in the other direction. 

 If hydrogen or some other inert gas is present, the band broadens symmetri- 

 cally at first. Larmor 2 has suggested that this unsymmetrical widening may 

 be due to the formation of loose molecular aggregates, which vibrate in longer 

 periods owing to this mutual influence. Wood and Guthrie 3 find that the 

 cadmium absorption band X 2288 broadens symmetrically in the case of 

 pure cadmium but very asymmetrically when mercury is present. A very 

 promising field for research is suggested by this work, one that will probably 

 throw much light upon the mechanism within the atoms themselves. 



Quite recently Dufour * has succeeded in obtaining the Zeeman phe- 



1 Astrophys. Journ., 26, 41 (1907). 8 Ibid., 28, 211 (1909). 



2 Ibid., 26, 120 (1907). * Phys. Zeit., 4, 124 (1909). 



