THE POLARIMETER 



477 



Transition Tint. When a beam of white light 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 yellow rays of white light through 90, and the remainder combine 

 to form a peculiar pale rose or lilac tint known 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, 18 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 x -f- a degrees, and in the other 

 half % a degrees where a represents the rotation due to the quartz. Owing 

 to the different rotations assumed by the components of white light, 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 activity 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. 



C 3 



FIG. 289 



FIG. 290 



Half Shadow or Penumbra Devices.* In Fig. 289, let A and B 

 represent the vibration planes of two beams of polarized light travelling 

 towards a nicol prism as analyser. Let D, OD l9 OD 2 , 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 . Similarly 

 in the position OD 2 all 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 AOB small, a nicol prism used as 

 analyser will on rotation through the small angle show three well-defined 

 positions, as indicated in Pig. 290. In position OD^ left half dark, right 

 half illuminated ; in position OD 2 , right half dark, left half illuminated ; 

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

 position, and is one of great accuracy ; it was devised by Jellett 19 in 1860, 

 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. f " A rhombic prism of Iceland spar, whose 



* " Half shadow " has come into use as the term defining these devices as a slavish translation of the Germaa 

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



t Most textbooks, evidently quoting from the same source of misinformation, describe as Jellett s aconstruction 

 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. 



