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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 One end A fits into the nozzle of the objective, from which the analyser is withdrawn, the latter now fitting into the other end B of this crystal- stage.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.