34S 



OPTICAL PROJECTION 



whose planes are at an angle of 45 with the plane of polarisa- 

 tion, the beam becomes circularly polarised ; and if in front 

 of this mica plate we place another quarter-wave plate which 

 can be rotated, the beam again becomes plane-polarised in 

 any plane we please, according to the position of the second 

 mica plate. Such an arrangement of two quarter- wave plates 

 can either be mounted in front of the polariser (the second 

 one in a divided circle with four spokes as usual), or a slot 

 may be provided before the slide stage, in which one mica is 

 placed mounted in a frame, and the other in a rotating frame 

 like fig. 195. 



The 'bright field' produced in this way is as good as 

 that of the Nicol polariser ; but if the two quarter-wave plates 

 are gauged as usual, by the sodium flame, the dark field will 

 show a perceptibly reddish tinge the ' tint of passage.' 

 This is not pleasant, and I consider it better to choose a mica 

 film very slightly thick, when the little residual colour will 

 be blue, which in the faint light will not appear perceptibly 

 inferior to a true black. 



197. The Rotating Frame. The greater part of sub- 

 sidiary apparatus will be best mentioned as required, but 



one item should be 

 mentioned here, as in 

 continual use. It is a 

 frame of standard size, 

 in which films or other 

 preparations can be 

 rotated whilst in the 

 slide-stage of the polari- 

 scope. This is effected quite simply by a small pinion gearing 

 into a circular rack ; and the preparations, which are cemented 

 in balsam between two glass discs the same size as those used 

 for slides mounted in wood, are kept from falling out of the 

 brass cell by a spring. Several of these rotators will be found 

 very serviceable, the cost being only about 4s. 6d. each. 



FlG. 195. Rotating Frame 



