Transparent Inactive Crystal Plates. 



185 



Fig. 7. 



ization of transmitted waves, was sufficient reason to discard it. 

 In the final arrangement adopted, no glass surfaces intervened 

 between the nicols and the crystal plate. An enlarged image 

 of the plate was obtained by means of a weakly magnifying 

 microscope consisting of the Bertrand lens and ocular, above 

 the upper nicol. 



Intense sodium light flame. — To increase the intensity of 

 the sodium light flame, and with it the accuracy of the observa- 

 tions, an arrangement was adopted which in practice has proved 

 entirely satisfactory. A 25 ce platinum crucible was filled with 

 a mixture of sodium chloride and sodium carbonate and 

 heated over a Bunsen burner, a special mounting of thick 

 platinum wire having been made for the 

 crucible as indicated in the diagram. A 

 wick of fine platinum wires carried the 

 molten salts from the base of the cruci- 

 ble out into a strong and constant blast 

 lamp flame, the high temperature of 

 which produced an intense sodium flame 

 which lasted for days, until the salts in 

 the crucible were exhausted. An oxy- 

 hydrogen blast was also tried, and 

 although it gave a much more intense 

 flame, its regulation proved troublesome 

 and there was danger that the platinum wick might melt down 

 unless the flame was constantly regulated. The fumes from 

 the salts were carried off under a hood. The microscope was 

 placed in a horizontal position and pointed directly at the flame 

 without intervening reflector. 



Results. 



Isotropic Plates. — Plane polarized light waves, in passing 

 obliquely through a mounted crystal plate, encounter not only 

 the boundary surfaces of the crystal plate, but also those of the 

 object glass, the cover slip and the Canada balsam in which the 

 plate is mounted. Each surface exerts a certain rotatory in- 

 fluence on the plane of polarization of the transmitted light 

 wave. In order to obtain an idea of the effect of the glass 

 plates and Canada balsam alone on the rotation of the plane of 

 polarization of a transmitted light wave, a blank glass slide 

 was prepared consisting of an object glass (n Sa = 1'5 11) and a 

 cover glass (n Na == 1/520) cemented together with Canada bal- 

 sam (%r a = 1-537). This slide was mounted on the universal 

 stage 1 whose different circles may be designated as follows 2 : 



1 This effect has been considered in some detail by G. Cesaro in the Bull, 

 de l'Acad. roy. de Belgique, Classe d. Sciences, No. 7, Juillet, 1906. 

 2 See this Journal (4), xxiv, 343, 1907. 



