544 



NATURE 



[July 24, 19 13 



features of the mercury spectrum. The main line is 

 accompanied by two satellites of greater refrangibility ; 

 But these are so close to the principal line, and are of 

 so much smaller intensity, that they do not diminish 

 appreciably the unique value of this line, which still 

 remains the most powerful source of monochromatic 

 light for work at the violet end of the spectrum. 

 A c t u a 1 measurements in 

 which the violet line has been 

 used, both with and without 

 the satellites, have shown that 

 the errors introduced bv the 

 presence of the latter do not 

 exceed one part in 10,000 on 

 the readings of a polarimeter. 

 This error would, therefore, 

 be quite inappreciable in the 

 ease of all readings of less 

 than ioo°. 



The visible spectrum does 

 not by any means exhaust 

 the usefulness of the mercurv 

 arc. The powerful series o'i 

 ultra-violet lines, which are 

 freely transmitted bv the 

 glass of the silica ' lamps 



coloured screens prepared from gelatine films stained 

 ivith suitable dyes. 



B. — Rotatory Polarisation. 

 The phenomenon of rotatory polarisation was first 



discovered in the case of quartz. Arago in 1811 (Mem. 



Inst., [811, pp. 93-134) found that a plate of quartz 

 interposed between a polariser 

 and analyser was capable of 

 depolarising the light in such 

 a way that transmission took 

 place where previously there 

 had been complete extinction. 

 When plates of suitable thick- 

 ness were used the trans- 

 mitted light was no longer 

 white, but beautifully tinted, 

 the colour of the light vary- 

 ing- with the thickness of the 

 nlate. Thus with increasing 

 thickness we have progres- 

 sively yellow, orange, rose- 

 red, violet, blue, and green. 



These colours were shown 

 by Biot to be due to a rota- 

 tion of t.he plane of polarisa- 



II 



Wave. -U?i£!h seal; o 

 Fig. 2.— Resolution of the 



nn PH n n 



ine by the echelon spectroscope. The actual dis: 

 diagram. (By courtesy of Prof. Stansfield.) 



*so 300 Willi -Afl^strom umt-i 



i of the components is shown by the 



(Fig- j)» ar e of value for a number of scientific pur- 

 poses, and have found an important technical applica- 

 tion in the sterilisation of water. 



At the other end of the spectrum, the magnificent 

 though invisible line at wave-length 10,140 has proved 

 to be of unique value as a starting point for calibration 

 work in the infra-red. It will also be remembered 

 that some of the longest waves of light that have yet 

 been detected were discovered by Rubens in the rad'ia- 

 tion from a mercurv lamp. 



tion, which increased (a) with thickness of the plate, 

 (b) with change of colour from red to violet. It is 

 therefore impossible when a beam of polarised light 

 has passed through a quartz plate to extinguish all 

 the colours simultaneously. 



The tints which Arago observed were due to the 

 selective extinction of light of different colours by the 

 mirror which he used as an analyser. This selective 

 extinction may be shown bv inserting a direct vision 

 spectroscope in front of the apparatus : the plate 



1 of mercury 



of Mtssrc. A. Hili>er, Ltd.) 



Resolution of the Mercury Spectrum. 



One merit of the mercury arc as a source of light 

 consists in the readiness with which the three main 

 components may be separated. A direct vision prism 

 of quite moderate dispersive power, placed in front of 

 the eyepiece of a polarimeter, produces a separation 

 of the three images which is sufficient for most pur- 

 poses. The lines may also be separated by' means of 

 NO. 2282, VOL. 91] 



which produces the pale yellow colour has rotated 

 the violet light through 180 , so that it is extinguished 

 exactly as if no quartz plate were present; the vellow 

 tint is the complementary colour to that extinguished. 

 As the thickness of the plate increases, the same effect 

 is produced with light of longer wave-length; as the 

 extinction moves from violet to red the complementary 

 colour changes from vellow to orange, red, blue, and 



