574 REPORT — 1891. 



fixed support so that the board can oscillate a little sideways. This carries the 

 image of the sun backwards and forwards over the slit, so that the light admitted 

 by the slit is alternately taken from a part of the solar disk near the following 

 limb which is advancing towards the earth, and from a part near the preceding 

 limb which is receding. Accordingly the solar lines in the spectrum are alternately 

 shifted a little — perhaps about one-twentieth or one-thirtieth of a tenthet-metre — 

 to the right and left, while the earth-lines maintain their position unaltered. The 

 eye readily detects this motion even when so small. 



There is a solar line a little less refrangible than the eighth pair of double 

 lines in the great B oxygen group, which, with the arrangement described above, 

 is seen to approach and recede from the double line in sympathy with the motion 

 of the pendulum. Another line well placed for the observation is the earth-line, 

 which is a very little more refrangible than D., ; and another convenient group is 

 where there is a strong iron line on the more refrangible side of D.,, about as far 

 from Dr, on one side as Dj is on the other. There is an earth-line in nearly the 

 same position. The two appear as a single line when the light is taken from the 

 preceding limb of the sun, and as a double line when it is taken from the following 

 limb ; and with the pendulum arrangement these appearances alternate. 



Of these three the observation on the B line can be well made in the second 

 spectrum of a Rowland's grating If inch long. The observations near D are 

 best made in the fifth spectrum. 



There are of course multitudes of other lines on which the observation can be 

 made. 



4. On the Cause of Double Lines in Spectra. 

 By G. Johnstone Stonet, M.A., RSc, F.B.S. 



The lines of the specti'um of a gas are due to some events which occur within 

 the molecules, and which are able to affect the retber. These events may be 

 Hertzian discharges between molecules that are ditterently electrified, or they may 

 be the moving about of those irremovable electric charges, the supposition of 

 which offers the simplest explanation of Faraday's law of electrolysis. The 

 amount of the charge which is associated with each of the bonds, and of which two 

 or more seem to be present in every chemical atom, is always the same quantity 

 of electricity. In a communication made to the British Association in 1874 the 

 author invited attention to this fixed quantity of electricity as one of thi'ee funda- 

 mental units presented to us by nature (see ' Phil. Mag.' for May 1881), and 

 estimated its value, which is about the twentiethet (i.e. 1/10-") of the electro- 

 magnetic unit of quantity in the ohm series. 



Several considerations (of which perhaps the most decisive is the phenomenon 

 of the reversal of lines) suggest that the source of the spectral lines is to be 

 sought not in the Hertzian discharges, but in the carrying about of the fixed 

 electric charges, which for convenience may be called the electrons. The present 

 investigation however is not dependent on this or any other particular hypothesis, 

 since it is with the laws of the events within the molecules that it is concerned, 

 and the course of the investigation shows that these laws are the laws of the 

 motion of separate elements of volume, which may be conveniently thought of as 

 the motion of those parts of the molecule to which the electrons are to be regarded 

 as indissolubly bound. An electron, if waved about in some particular way by 

 the motions within the molecule, would occasion such electro-magnetic waves as 

 are revealed to us by the spectroscope. 



Now the irrotational motion of an element of volume consists in its traversing 

 some orbit, accompanied perhaps by a simultaneous distortion of its form. We 

 are only concerned with the orbital motion. This motion may be resolved by 

 Fourier's Theorem into the superposition of partials, each of which is a simple 

 pendulous motion in an ellipse, and each of these partials produces its own line in 

 the spectrum. Seven constants are required for the full determination of each 

 partial if the orbit of the electron is a curve of double curvature, or five if it is a 

 plain curve. Now the observation of a line supplies only two equations between 



