398 Prof. Silvanus P. Thompson on 



plate, and the eye, instead of trying to determine the absolute 

 degree of blackness, had merely to make a comparison be- 

 tween the tints of the two simultaneously visible halves of the 

 visual field. With the same object Pohl*,in 1856, suggested 

 the use of a thin piece of mica covering part of the visual 

 field. This suggestion was improved upon in 1874 by 

 Laurentf , who proposed to cover half the polarized field with 

 a half-wave plate of quartz ; this construction being known 

 as the saccharimetre a pdnombres, or half -shadow polarizer. 

 In the Laurent apparatus, the two halves of the visible field 

 are consequently polarized in planes inclined to one another 

 at an angle dependent on the position given to the half-wave 

 plate. 



In 1860, Jellett % introduced the ingenious triple image 

 prism, in which, at either side of an ordinary image, are seen 

 two extraordinary images, having their respective planes 

 of polarization inclined at a small angle to one another. 

 Hence, when one of these two images is at maximum extinc- 

 tion, the other will not be quite extinguished ; and as a very 

 small angular displacement will greatly affect the relative 

 amounts of light in these two images, the adjustment to 

 equality by the eye gives the exact position of the mean plane 

 of polarization of the two images within a very narrow range. 

 In Jellett's apparatus this prism was used as an analyzer 

 only. 



Based upon this idea, Cornu§, in 1873, suggested a prism 

 a penombres, constructed by Duboscq, consisting of a Nicol 

 prism, which, having been cut in two longitudinally, in a 

 plane at right angles to the balsam-film, and a small wedge 

 ground away, was reunited with balsam, so giving in the two 

 halves of the visible field light polarized in planes at a small 

 angle to one another. In Cornu/s apparatus, this divided 

 Nicol prism was to be used as analyzer, with an ordinary 

 simple polarizer. Schmidt and Haensch||, in 1878, simplified 

 this construction by cutting out the wedge from one half of 

 the Nicol prism only, and again reuniting the parts. 

 LippicMl", in 1882, suggested for the same purpose covering 

 half the polarized field with a smaller polarizing prism rotated 



* Wiener Berichte, xxii. p. 492, 1856. 



t Journal de Physique (1) iii. p. 183, 1874. See also Dufet in Journal 

 de Physique (2) i. p. 552, 1882. 



X Rep. Brit. Assoc. I860, ii. p. 13 ; Proc. Roy. Irish Acad. viii. p. 279, 

 1863 ; Trans. Roy. Irish Acad. xxv. p. 371, 1875. 



§ Bulletin Soc". Chim. [2] xiv. p. 140. < 



|| See Landolt, Bericht iiber die Polar isationsapjmr ate, Berlin, 1880. 



% ZeitscJiriftfur Instrumentenkunde, ii. p. 176, 1882 ; Wiener Berichte, 

 xci. p. 1059, 1885. 



