22 



Mr. J. N. Lockyer. 



[Dec. 18, 



author in 1864, in which the planet's spectrum is observed or photo- 

 graphed together with a daylight spectrum. These photographs show 

 no sensible planetary modification of the violet and ultra-violet parts 

 of the spectrum of the planets Venus, Mars, and Jupiter. 



Numerous spectra of small areas of the lunar surface have been 

 taken under different conditions of illumination. But the results are 

 negative as to any absorptive action of a lunar atmosphere. 



IV. " On a New Method of Spectrum Observation." By J. N. 

 Lockyer, F.R.S. Communicated at the request of the 

 Committee on Solar Physics. Received December 10, 1879. 



In anticipation of my report on the Methods of Mapping Spectra, 

 which I have been requested to prepare for the Solar Committee, I beg 

 to present to them the following account of some recent work which 

 has been suggested during the preparation of that report. In the 

 " Phil. Trans." for 1873* I gave an historical account, showing how, 

 when a light source, such as a spark or an electric arc, is made to 

 throw its image on the slit of a spectroscope, the lines had been seen 

 of different lengths, and I also showed, by means of photographs, how 

 ^ery definite these phenomena were. It was afterwards demonstrated 

 that chemical combination or mechanical, mixture gradually reduced 

 the spectrum by subtracting the shortest lines, and leaving only the 

 long ones. 



On the hypothesis that the elements were truly elementary, the 

 explanation generally given and accepted was that the short lines 

 were produced by a more complex vibration imparted to the " atom" in 

 the region of greatest electrical excitement, and that these vibrations 

 were obliterated or prevented from arising by cooling or admixture 

 with dissimilar atoms. 



Subsequent work, however, has shownf that of these short lines 

 some are common to two or more spectra. These lines I have called 

 basic. Among the short lines, then, we have some which are basic, 

 and some which are not. 



The different behaviour of these basic lines seemed, therefore, to 

 suggest that not all of the short lines of spectra were, in reality, true 

 products of high temperature. 



That some would be thus produced and would therefore be common 

 to two or more spectra we could understand by appealing to Newton's 

 rule : " Causas rerum naturalium non plures admitti debere quam 

 qua3 et verse sint et earum phasnomenis explicandis sufiiciant," and 



* " Phil. Trans./' 1873, p. 254. 



f " Proc. Eoy. Soc," vol. xxriii, p. 159. 



