THE POLARIMETER 



477 



Transition Tint. — When a beam of white hght is passed through an active 

 material, each of the several spectral components is rotated through a 

 different angle, and in a system made up of parallel nicols and interposed 

 active material those rays rotated 90° do not reach the eye of an observer. 

 A plate of quartz cut perpendicular to the optic axis and 3-75 m.m. thick 

 rotates the }-ellow rays of white light through 90°, and the remainder combine 

 to form a peculiar pale rose or lilac tint kno\\-n as the transition tint. The 

 appearance of this tint forms a critical position. The device used to obtain 

 the tint is the Soleil bi-quartz,i« which is made up of halves of dextro- and 

 levo-rotatory quartz. Such a plate interposed between parallel nicols gives 

 a uniform field of a pale rose tint. Now let a rotation of x° be introduced. 

 In one half of the field the rotation will be a; + a degrees, and in the other 

 half X — a degrees where a represents the rotation due to the quartz. 0\\ing 

 to the different rotations assumed by the components of white Ught, the 

 colour effect transmitted on either side of the field is different, on one' side 

 green rays and on the other red rays predominating. The critical position 

 again appears on the interposition of a rotation of — x°. If the quartz plate 

 were wholly of the same optical actiWty the transition tint would again 

 appear at the same position, but then there would be no sharp contrast at 

 positions a little removed from the critical position. 



f, CB 



Fig. 2S9 



Fig. 290 



Half Shadow or Penumbra Devices.* — In Fig. 2S9, let A and R 

 represent the vibration planes of two beams of polarized light travelling 

 towards a nicol prism as analyser. Let D, OD-^, OD^, represent various 

 positions of the optic axis of the analyser. Thus in the position D^ per- 

 pendicular to OA the analysing nicol is crossed with reference to OA . Sindlarly 

 in the position OD^ aU light vibrating in the plane OB is eliminated. When,, 

 however, the position OD perpendicular to OC bisecting the angle between 

 the vibration planes is assumed, equal amounts of light are transmitted 

 from either source. By making the angle AO B small, a nicol prism used as. 

 analyser vnH on rotation through the small angle show three weU-defined 

 positions, as indicated in Fig. 290. In position ODj left half dark, right 

 half illuminated ; in position OD2, right half dark, left half illuminated ; 

 in position OD, equal illiunination throughout. This last is the critical 

 position, and is one of great accuracy ; it was de\'ised by Jellett^^ in i86o^ 

 for whom the first half shadow prism was made by Bryson, of Edinburgh, 

 and the first half shadow polarimeter by Spencer, of Dublin. 



Jellett Half Shadow Device.^ — " A rhombic prism of Iceland spar, whose 



• " Half shadow " has come into use as the term defining these devices as a sla\Tsh translation of the German- 

 " Halbshatten." Though clumsy, " isophotostatic " would be a better word. 



+ Most textbooks, e\'idently quoting from the same source of misinformation, describe as Jellett's a construction- 

 quite different from that given by the Irish physicist. They also fail to state that he located the half shadow.' 

 device in the analyser and not in the polarizer. Jellett's exact wording is quoted above. 



