ArKir, 27, 1899 I 



NA 'rUR/i 



607 



two millions sterling. Il rciiiimls one of llic iiilliKMn c ol 

 «lectric light on gas undeilakings some twenty years ago. 

 ■Gas stock depreciated llicn many millions. Now they 

 never were so high in value. There was some excuse 

 then. The electric light was a great boon. It was a 

 dangerous rival. There is no excuse now. Nothing 

 whatever about wireless telegraphy has given the smallest 

 indication that it is going to supplant wires. It simply 

 fills a want. Communications between li|j;htshij)s and 

 shore, between ship and ship, between moving shi|>s anil 

 shore, between isolated lighthouses and the mainlanil, 

 become possible. They were not so before. 



Messages between the .South I'oreland and Moulogne 

 have been sent. There is no reason why they should 

 not. Mr. I'rcece, many years ago, intliiated how il 

 could be done. Mr. Marconi has ilonc it in anolli(!r 

 way. IJut an isolated experiment, however successful, 

 and a single circuit, though it transmitted messages ai • 

 curately at the rate of twenty words a minute, is not 

 ^oing to reiilacc one of the present submarine wires, each 

 of which (an transmit similar messages at 600 words 

 a minute. Wireless telegraphy may maintain comnumi- 

 (alion with outlying islands when cables break down. 

 It did so by the I'ost Office with the island of Mull in 

 1895, before Mr. Marconi was heard of; but it is not 

 going to replace one single cable between (Ireat liritain 

 and the continent. Marconi's system has now been 

 before the public for nearly two years, but w(^ have not 

 heard of anything new from a scientific point of vi(!w 

 since it was first |)ublished. The last report is that il 

 is possible to direct the signals to one selected point. 

 Two years ago it was said to be able to do the s.uiie 

 thing by tuning. The fact is that we have in these re 

 peated sensational experiments a pure scicntifii: appa 

 ratus boomed by energetic financial speculators for their 

 own individual gain, and not for the benefit of the public 

 — the worst feature of this money-grubbing age. 



yy/A AnCHELSON ECHELON SPECTRO- 

 SCOI'E. 



SIN( l'^ I'rof. Mirlielson's announcement of his new 

 form of si)ectros( ope, in the Aiiicricnn /miniii/ <// 

 St/f/Ui; March iH'jS, and Ash(i-I'hysii:iil Jaiininf, 

 vol. viii. p. 37, 189X, all having any connection with 

 spectroscopic work have been waiting with great interest 

 to see the performance of the instrument. This curiosity 

 lias now been satisfied in a most complete manner, an 

 echelon of fairly large dimensions having been success- 

 fully constructed by Mr. A. Ililger, of Islington, which 

 the writer has had tin; jilcasure of examining, liefore 

 describing this, it may be usefiil, for the benefit of many 

 who have not noticed the previous reports, to briefly 

 state the characteristii s of the new spectroscope. 



In an ordinary dif'fra<.tion grating, consisting of eipii- 

 distant lines ruled on a plate of glass or speculum 

 metal, tlie resolving power is determined by the product 

 {m n) of the total number of lines in) and the order of 

 spectrum observed (///;. As in this type of grating the 

 succeeding orders after the first decrease in brightness 

 very rapidly, little progress has been made in the 

 cncleavour to increase the order observed. Many 

 Jittempts have, indeed, been made to concentrate the 

 light in one of the first three orders by means of special 

 adjustment of the ruling, so that higher magnifications 

 could be used, but with little or no certainly or e(|uality 

 of result. If, however, insl(sid of obtaining th(; phase 

 difference by alternations of opaque and transparent 

 spaces, the necessary retardation is brought about by ,1 

 progressive damping, as it were, of llie wave front by 

 increasing thicknesses of an absorbing transparent 

 material, it would seem that it is possible to throw prac- 

 tically tlie whole of the transmitted light into any one 



NO. 1539, VOL. 59] 



order. In such :i < ase the; grating space must be pro- 

 portionately iiK leased so that il remains coiiinuMisuiale 

 with the degree of the order ; M\i\ as the lines would 

 need to be made with no more ac curacy than before, the 

 grating could be completed in less time, and therefore 

 with less chance of ileforinity due to temperature and 

 other changes. 



Instea<l, however, of attempting to rule lines on glass 

 or metal, wlii( h would be an extremely (lilficull matter 

 for this iiurpose, I'rof Michelson took an entirely original 

 step by iMiilding up llie spacc-s with ;i number of strips of 

 glass having optically plane surfiices. The app<;.irancc 

 presented by such a dispersive arrangement will then be 

 as shown in the following diagram (Fig. 1), th(: arrows 

 indicating the directions of the incident and transmitted 

 rays. 



I''t(;. I. -SliowiiiK piilliH of direct niul (llfTraclotl 

 riiyn llniiM|{h llic echelon. 



"The int(!resting feature of the new arrangement is 

 the smallness of the number of elements necessary to 

 give results which may be comparable to those given by 

 the best ordinary gratings. This can be simply shown 

 as follows : Let nli - .r, and hd I be the surfaces of 

 one of the steps between two of the parallel plates of 

 the echelon. If then in is the order -of spectrum to be 

 observed, we sh.ill have 



Therefore 



IIIK f^.l li 



dill I sin i; I 



Ja 



If M is the displai emeiit corresponding to RX, and M, 

 is that corresjjonding to iini 1, then assuming Caui hy's 

 formula, 



M « I- ^'„. 



and taking as a first approximation 



III ill 1) , 



we have 



»tf| L Ja A 



" Kormostspecimens of flint glass the coefiii icnt ii{ in 

 the last expression is apjiroximately eipial to unity, so 

 that if — 'oor, say, as in the case of the two yellow 

 sodium lines, .-iiid / 5 iiiiii. \(),(xir> A, then 



,/(l i(i</n, ; 

 that is, the two sodium lines would be seen separated 

 by ten limes the distance between the successive spectra. 



