288 EEPOBT— 1880. 



the more complete molecule. It would not be unreasonable to suppose 

 that a similar cause prodnces also the other cases of continuous absorption. 

 But Prof. Stokes ' has suggested another cause which may produce such 

 a continuous absorption : — 



'We have reason to believe that the mere motion of matter through 

 the ether is insufficient to produce vibrations. There must be two portions 

 of matter exerting forces on each other in order that the ether should 

 be thrown into agitation. In ordinary line-spectra we consider that 

 the two portions of matter form part of the .same molecule. Now, it 

 seems possible that also two portions of different molecules should in 

 their rapid approach towards each other, or recession from each other, 

 cause forces in the ether which produce vibration. These latter vibrations 

 we might exjiect not to take place in fixed periods, but to produce what 

 we call a continuous spectrum. We may suppose that at the lowest 

 temperature at which, for instance, oxygen is luminous, the vibrations in 

 the ether are chiefly produced by this rapid relative motion of different 

 molecules, while at higher temperatures the relative motions of different 

 portions of one molecule might have the upper hand; the continuous 

 spectrum in one case, and the line spectrum in the other, might thus be 

 explained.' 



IV. The Method of Long and Short Lines. 



If the spectrum of a metal is taken by passing the spark between two 

 poles in air, the pressure of which is made to vary, the relative intensity 

 of some of the lines is often seen to change. Similar variations take place 

 if the intensity of the discharge is altered, as, for instance, by interposing 

 or taking oat a Leyden jar. It is a matter of importance to be able to use 

 a method which in the great majority of cases will give us at once a sure 

 indication how each line will behave under different circumstances. This 

 method we now proceed to describe. 



It has often been stated, even by the earliest observers, that the 

 metallic lines when seen in a spectroscope do not always stretch across the 

 whole field of view, but are sometimes confined only to the neighbourhood 

 of metallic poles. Some observations which Mr. Lockyer had made jointly 

 with Prof. Prankland '^ led him to conclude that the distance to which 

 each metallic line stretched away from the pole could give some clue on 

 the behaviour of that line in the sun. In the year 1872 Mr. Lockyer ^ 

 worked out this idea and obtained important results. 



In his experiments an image of the spark was formed on the slit of the 

 spectroscope, so that the spectrum of each section of the spark could be 

 examined. Some of the metallic lines were then seen to be confined 

 altogether to the neighbourhood of the poles, while others stretched nearly 

 across the whole field. The relative length of all the lines was carefully 

 estimated. Tables and maps are added to the memoir. 



In order at once to clear up a widespread misapprehension we may 

 mention that the longest lines (that is, those which stretch away furthest 

 from the pole) are by no means always the strongest. Even spectro- 

 scopists use the terms, longest line and strongest line, sometimes as 



' See note to paper by Schuster ' On the Spectrum of Oxygen,' Phil. Trans, clxx. 

 p. 38 (1879). 



2 Proc. Poij. Soc. xviii. p. 79 (1869). 



3 Phil. Trans, clxiii. p. 25.3 (1873.) This paper will also be found to contain 

 references to previous observations bearing on the subject. 



