3^ 



fomisiMf « 



NATURE 



r «f attadc for aU tfaoe Mn>km i f«r 

 llMt of dK^ifrKtioa flraCMif. 



** in andi a wi^ as to add to 

 or to aamd eadi odker*s 



Kwwwiiaiy, the «uiu w J y jriMe 



TlRve cae l»« made a ocura toly paraflei, 



ipvt can be varied at «9L Wlm Gg|ht b urie c ted fran 



dw vmtm t m «1tfcii face «adi odier. ^e two r efccted beans 



of Sf^ wws 



eacn oUMt, 



Tbe atenMifMWs of fi^|ht aadl dariOMK vWdi oocor arbea 

 the eve o b n ei^we * m dw di rectio n of the normal are verr 

 awrfeed so %img as the pfanes wn rtrj near li ^ e et h e r ; hit 

 ai. ti» Citawn i a aease* the iotarferenoes becnme leas and 

 les» dntinct, mtta at a dirtance . aAidk defwadt «• Ae 

 timrmcitr «4 the ja ci i m l Kfbl, di^ vamrii oomnleieiy. A 

 perlecthr definite relatioa hoUs bUatJt a the ^ vhOS Bty 

 curve "' and the ir hararter of die ra d iati on, so that dK 

 one can be dedaoed from the other. 



Xonr the ** resohrang pon^ *' of soch an apparatas is 

 OMAsanvd bv the mmdier of B^^ awves m dte donbled 

 distance betm^een the sorfaoes. This is alioiat loogooe lor 

 m distance of i iach; bat the £stsaoe is, in fact, 

 aMMMled, and as the mstmment rtaeif is ptacticaBv' free 

 from errors of anj sort, rts resolnaig power is |aacticallv 

 unfimited. 



The use of diis m etho d of Q^bt-«ave anairsis tt attended 

 vith certun ififficahies, and dw resides o b tained are not 

 alvajs free Irom uncertaaides ; bat a riem- of dK fact 

 diat at tins tone no odxr M R tho di > of ibis power had been 

 d r r is e d , k has amptr faoved te osefidaess. Amoii^ the 

 rcsohs aclae««d bj it nun- be mantiomd the itioiafion <if 

 manr fines s up pos ed siag^ into donbhli. ipiadnykUi. Ic ; 

 die measuvenneat of then- ^^vm-n apart; ^Ae i B tlii bn ti oa 

 of fi^ in the oomponems; die measm«ment of their 



viAdh and the chsuBts p rodnced in them br 

 press ur e, and p»^J^nu■ «rf a msgnrtit field. 



Arao^ the rad i a ti an s thns tTsmimd. one proved to he 

 so oearlv h o rooeu a u ans that more thm >Qo,oao iacer- 

 fcrence buids cmdd stS be o bserwed. Oth e iaiMt ex- 

 pressed, ^e exact aamber of %jht ««nes ia a ^m» 

 distance, sanr lo cart., conid alnajA be ~ 

 In- a comparison vidi ihe standmd aaefter the 

 vrsTeOes^di of tUs rafiadon cooid be 

 to serve as a basis for d vav«4HH^dB. 



The standard of ki«A itseV, the 

 de&ied as the £stance bux^n two 

 affld notvidhstnadiae aM the 



and presermdoa, dasc is no assoraaoe dot it is not 

 0oii^ a ronsta n t shnr rhsngi. doa l illesi wxj 

 perhaps j pp ieiisb i e bv the 1 1 lini na mil ii of 

 loflical method if Aere mere amr 

 ;^e slandai d widb vdnth it coadd he 

 earth V circamfereace vas snpposed to be socft a 

 and the metre was wTjinslr 



of an ear^-^foadnaat ; brnt'dv Tansms mmh minamii of 

 diis qaadrant Tsried so mnch &at dw 

 The «ttamnt to base Oe stamdrnd on Ae hmBdi of 



Bat «v Snve aov dK aseaas of 

 metre vidi dK l-^<diof a !i^ m*i« |fte 

 «"«mas 1>SSS><^3 arnwes of dhe red 

 cadnasm vapoml. so dua ateald the 

 hmt or desui uyed , or dhmdi it vnrr in ha^ft in Che 

 of jranrs. its o^jpnal valor cna be juxa a uul so 

 Aat no mjc raacnpe ta nH^Amct tfbe «immt Tt»c it wi 



*^*** H l."iJ " ye^gi fcanelnsi mfci ul m^ p i Bl i bm 

 ""^^ ^ f"'!'^*^ ^**^ ^^ piiiMimi off spectnaesf^, it 



*» ^'fMtadfi B ov remtaidBd. Tim need ams sop^nd Inr 



ptascs of exncxhr eqol thirhnrwand fm maiLii kmdaf 

 i<mrsw vshmoeits mnne. 

 TV iralmg ads hr 



"Kavr truns of vcbach an 



XO- 2202. VOL. 88] 



(January ii, 1912 



hm than ata. dK dJUance 



of wavM (ttsaa% 

 the ^r>tim[ 

 vravcs), a hece a* diis letaida l i on in Ae fchehm 



■may 



BuK the reaoh-if^ pomer dtpandi on the IMal 

 of the estraaae raja, and this amy be made very hwm> either 

 bv having an enoinio as nomber of flrnnats win tam> 

 retardacians, or by a nwiiparativtlf anwB OMnher of 

 deaMats widi har^ tctaidatiaas. For example, an echrioe 

 of tkmtf platot of gh)» i Inch dnch, onch pradwdim a 

 reCardatHW of t^jotMi wavm« manid have a reaofto^^ paaivr 

 sn tiama dmt of die grating: aad t^> 



Pettit and Fabry dwwad dart bv the 

 ftpurttd ne l ecti o ns b e t ween tw silvered aarfacm* a 



m(h reaoivmg pow<er is obtained, and a few jcars 

 Lanoner diviwid dK plate inlet fci oaaeter, a hith 

 pi a tti t aHj the same mea. 



The r eso l v iag power of dD of these nearer devices is 

 chait y mnav tioHS as great as that of the jjratiag; bm aS 

 iipiaBjtbm the ob fecti o n wluch holds (bm to a far km 

 eslent) for the grating that the diferent s oci-wding 

 spectra overlap- It s true that this dificnhf may be over- 

 ooaae (diongh with some loss of 

 loss of hC^^t) bjr cmpiajfim 

 ubilnnB. mc^ and in dUslarm aH 

 aaencs have oomribnlcd iwllj of fiar-readang 

 which vrooid have been impomA ie widi die older btstra- 



advaacages in 

 that at B even 



for sach 



The diCiaLtion ipatliig 

 saapfidtr and oaaacmeace of 

 ia mdesence to 



poarar. Bat has the resolvmg pomer of 

 been pashed to the Bant? We have seen dot dris 



on dK anaaher of luiituss; aad it is certaariy 

 ^a Hcraase thas naw^aer* Bat ^he ^^T**^f^TT^T vaaie 

 IS only naciad if the rafiags are wery ar'rwaaily spaced; 

 for aastancOf the resolvaig power of the Rawiaad. 



K oady one4hBd of its theoretical vrine. This is a dSaect 

 rona ^ minre of inaocoracaes in ihe anndng of dK 

 If a gaUn g oaaM be oonslrwcted of, say, ssbwooo 



aridi eoKt spacing, the resolving power aandd be e^ 

 dmt of dK aaost pn a mfal erhehm The prnbhm of the 

 of s 



has had a fmr 

 of sta beltar icsadb ia dK 

 The eawvuiai organ in al mfiag 



dK screw, ahaJi ^aotws the uplicat 



eqnal places of dK order of a ^mflh tB>_ i e Budh of 

 each stroke: and dK pnaopnl 

 of «he m a rhi a e is to amhe the 

 dmt the errors aiie 

 of a 



*y » 



_ which is Ae 



the screw. A _ _ 



camU be prepared ia a few tnedcs. 



nhiih rahs 6-inch gratiags, reipmed tww years or 



and a screw shiih is to rale a graciag 15 ~ ~ 



be <UH p u>»ud to take a 

 fact. 



I manr be p.imiilid to stale a few of dK 

 cnoaanaaed 



if mr 



