440 



NATURE 



[March 9, 1899 



T)T>e I. 



Where large numbers of books have to be cared for, as in 

 libraries, " silverfish" and cockroaches are, in this climate, the 

 •enemies perhaps most to be dreaded. These lurk and breed in 

 the spaces between the woodwork and the walls, and in crevices 

 and crannies about the shelves. 



Mr. J.J. 1'letchi.r, Secretary to the Linnean Society of N.S.W., 

 ■has eliectually rid the library of the Society of the.se pests by 

 •freely dusting into the crevices ordinary powdered Paris green. 

 Wherever there was a space in which the insects could lurk, Mr. 

 Kletcher applied the powder, and now the books, which were 

 formerly much disfigured by the in.sects nibbling the bindings, 

 remain entirely free from damage because of the extermination 

 ■of the pests, Thos. Steel. 



Colonial Sugar Refinery Company, .Sydney, January 31. 



Radiation in a Magnetic Field. 



In the very interesting summary of our present knowledge of 

 the Zecman effect, Mr. Preston has interpreted a number of 

 results I obtained by the interferometer and the echelon spectro- 

 -scope in such a way as to cast a general doubt over the perform- 

 ances of the.se instruments. Some of these 

 interpretations are undoubtedly the result . 



of misunderstandings due to my own want 

 of clearness. I venture, therefore, to 1 



present a summary of the principal results, 

 emphasising where necessary the points I I 



which require further explanation. r-' S'> 



In the article to which Mr. Preston « 



refers {Phil. Mag., vol. xxxiv. p. 280, "■ 



1892), it was shown that the visibility 

 curve, in the case of about twenty radia- 

 ons examined, showed peculiarities from 

 which the character of the spectrum coulii 

 be inferred, and a considerable number of 

 "lines" were shown to be double, triple, 

 or more highly complex. Mr. Preston 

 ■remarks: "This structure has never yet 

 been observed by means of any ordinary 

 form of spectroscope, and accordingly it 

 .has been suggested that it does not exist 

 in the light radiated from the source, but is imposed on the 

 spectral lines by the apparatus used, namely, the interferometer." 



It might be replied that such an explanation would be very 

 difficult to accept, in view of the very great constancy of the 

 results, with instruments of different construction and dimen- 

 sions, with different observers, and with different forms of 

 vacuum tubes employed. But, if I am not mistaken, the only 

 attempt at explanation of the peculiarities of my visibility 

 curves, was that which attributes them to difil'raction effects ; 

 which, however, would necessarily be of a totally different 

 character, and indeed in most cases entirely insignificant. 



The real reasons for the absence of confirmation of these 

 results by the spectroscope are probably that the resolving 

 power is insufficient ; or where the resolving power is insufficient 

 the radiating substance is not in a vacuum tube, and the con- 

 sequent broadening of the lines under atmospheric pressure is so 

 great as to mask the details of structure ; or, finally, if the 

 substance is placed in a vacuum tube, the light is not sufficiently 

 ■bright. 



Turning now to Mr. Preston's criticism of the results obtained 

 in the investigation of the Zeeman elTect, the following remarks 

 may help to clear up the " most surprising statement that the 

 separation of the lines in the triplets produced by the mag- 

 netic field is independent of both the spectral line and the 

 substance." 



The law referred to is stated thus : " The separation is pro- 

 portional to the strength of field, and is approximately the same 

 for all colours and for all .substances." 



But on p. 137 of the same article (Aslrophysical fournal, 

 vol. vii. No. 2, 1898) will be found the further modification: 



" The following table shows that the law ... is only 

 approximately true. In fact, owing to the complexity of the 

 spectra, there is considerable latitude in the choice of the 

 distance between the outer groups. If this correspond to the 

 brightest components the law can hanlly be said to hold at all ; 

 but if the distance be taken between the centres of gravity of 

 the light areas, a fair agreement is found. The table gives 

 se|j.iration in tenth-meters for a field 10,000. The lines marked 



with an asterisk are less accurate than the others on account of 

 broadening : 



"Taking into account the uncertainty alluded to, the results 

 show on the whole a fair agreement, from which it may be con- 

 cluded that the separation is independent of the radiating sub- 

 stance and of the colour." 



A number of r.idiations have been examined since the fore- 

 going was published ; and while there are undoubted exceptions 



Typcn. 

 , plane of polar 



/\>;A,aI,"\/\^ 



Typ«ni. 



n, Equatorial. 



to the law, I still think, keeping in mind the limitations 

 referred to, it is in tlie main correct. 



These results and others obtained by the interferometer, as 

 Mr. Preston states, have been verified by the echelon spectro- 

 scope ; and I think the explanation of apparent differences be- 

 tween results obtained by these methods and by other forms of 

 spectroscope are to be explained, not by an actual variance, but 

 by a misunderstanding of the degree of approximation the results 



Hlue. ]\ W y^ ^ / ^-^ Type It. 



NO. 1532, VOL. 59] 



are intended to show, except where the resolving power of the 

 other spectroscopes is insufficient. 



Mr.jPreston states : " With apparatus which reveals structure 

 or multiplicity in the ordinary spectral lines, it is to be expected 

 that multiplicity would be readily revealed in the constituents 

 produced by the magnetic field ; yet in the case of .some lines, 

 the amount of finer structure revealed does not appear to be as 

 great as that observed with a good grating. ..." In reply to 



