POLARISED LIGHT 377 



With convergent light it is of course different. For the 

 simpler polariscopes a crystal-stage is provided, which consists 

 of a tubular fitting like fig. 208. One end A fits into the 

 nozzle of the objective, from which the analyser is withdrawn, 

 the latter now fitting into the other end B of this crystal- 

 stage. In the centre at s is a slot or stage with springs, large 

 enough to receive the crystal sliders, which are about an inch 

 wide ; and the stage is so placed that the crystal occupies 

 the spot where the cone of light is of the smallest diameter. 

 With the low convergence from the usual power alone, 

 the convergent (or divergent) light passes through the 

 crystal and analyser straight to the screen, and needs no 

 focussing lens whatever, the fringes appearing simply as 

 shadows. The rings and brushes are shown as perfectly in 

 this simple and inexpensive way, 

 as in any other ; but the choice of 



s! 



bi-axial crystals is limited, as only 

 very small angles will allow both 

 systems of rings to be brought into 

 the field together. Practically, only 

 nitre, cerussite,glauberite, and some 

 small-angled adularias are available, on this account ; uni- 

 axials of course are all available. It is customary to pre- 

 pare plates of other bi-axials cut across one, axis, to show one, 

 of the systems of rings ; but beyond one or two, such are of 

 little interest. 



With the optical arrangements shown in fig. 190, however, 

 any angles up to 60 or 65 can be shown. 1 The system of 

 converging lenses c is inserted in the nozzle, and the rays after 

 crossing in their focus and becoming divergent are collected 

 by the second system. This refraction has however been too 

 violent to project defined figures by mere shadow fringes ; and 



1 By interposing cedar-oil, as used for homogeneous immersion objectives, 

 between each face of the crystal and the adjacent lenses, larger angles can 

 be collected if deemed necessary. 



