484 CHAPTER XXIV 



Biot 2 P. Mirror affording polarized ray by reflection. A . Bi-refractive 

 achromatized prism compensating by rotation, extinction of the extra- 

 ordinary ray forming the critical position. Fig. 304. 



Mitscherlich 15 P. Polarizing nicol. A. Rotating nicol serving as 

 analyser and compensator, used with total darkness as critical position 

 with monochromatic light and with appearance of red field as critical 

 position with white light. Fig. 305. 



Robiquet 52 . P. Polarizing nicol. B. Soleil biquartz. A . Rotating nicol 

 serving as analyser and compensator. Used with transition tint as critical 

 position, and with white light. Fig. 306. 



Soleil- Duboscq. P. Polarizing nicol. B. Soleil biquartz. A. Analysing 

 nicol. C. Quartz wedge compensator. D. Colour compensator consisting 

 of a nicol prism and a plate of quartz. This instrument was designed for 

 white light and used 16-35 grams as normal weight. It was the first instru- 

 ment of a high degree of accuracy. Fig. 307. 



Soleil-Ventzke-Scheibler. In the hands of German opticians this last 

 instrument took on the arrangements seen in Fig. 308, the only changes 

 being the position of the colour compensator and of the normal weight to 

 26 048 grams. 



Jellett? 1 P. Polarizing nicol. E. Levo-rotatory material of known 

 rotation. F. Graduated telescopic tube to contain sugar solution, the adjust- 

 ment of the length of which serves to compensate the rotation due to E. 

 A . Analysing half shadow prism. White light was used with this instrument. 

 Fig. 309. 



.'- Wild 32 . P. Polarizing prism also serving as compensator by rotation. 

 G. Savart polariscope. H. Crossed spider lines. A. Analysing nicol. 

 This instrument was designed originally for monochromatic light and by the 

 addition of the Soleil-Duboscq compensating system becomes adaptable for 

 white light. 15 The normal weight is 10 grams. Fig. 310. 



Cornu 20 . P. Half shadow prism. .4. Analysing prism also serving as 

 compensator by means of rotation. Designed for use with monochromatic 

 light and 16-35 grams normal weight. Fig. 311. 



Precisely as the Soleil-Duboscq instrument with a changed normal 

 weight became in the hands of German firms, the Soleil-Ventze-Scheibler, 

 so this, with the additional change of the quartz wedge compensating prism, 

 became the standard design of German houses. 



Laurent 22 . P. Polarizing prism, in the older designs Foucault's modifica- 

 tion of the nicol being used. /. Half wave plate of quartz. A . Analysing 

 nicol serving as compensator. In this instrument the half shadow angle is 

 capable of adjustment by rotation about its longitudinal axis with variation 

 in the sensibility and amount of light admitted. Many instruments are 

 sent out with quartz wedge compensation system in addition to the compen- 

 sating analyser. Fig. 312. 



Trannin 28 . P. Polarizing prism. /. Senarmont polariscope arranged 

 at zero of scale so as to be out of adjustment equal to the rotation produced 

 by 10 cm. layer of a 10 per cent, sugar solution. F. Telescopic graduated 

 tube serving as compensator through adjustment of length. Fig. 313. 



Duboscq and Duboscq 29 . P. Polarizing nicol. /. Senarmont polariscope. 

 C. Quartz wedge compensator. A. Analysing nicol. Fig. 314. 



