382 OPTICAL PROJECTION 



each other by an aliquot part of a circle, found that all the 

 phenomena of quartz, either in parallel or convergent light, 

 are perfectly reproduced. Such a preparation treated as a 

 crystal gives the crystal fringes of quartz exactly, and two 

 of opposite rotations superposed, give Airy's spirals. 



Plates of mica of equal thickness crossed at right angles, 

 after Norremberg, as the plates become thinner and more 

 numerous, give a gradual transition in convergent light from 

 bi-axial rings, to the circular rings and four-armed cross of 

 a uni-axial. Two crossed give of course the four rings of a 

 crossed bi-axial ; twenty-four of about J wave thickness, give 

 effects quite undistinguishable from those of a calcite crystal. 



Plates of mica of various thicknesses crossed in different 

 ways 1 give very beautiful and complicated figures in con- 

 vergent light, which may still more be varied by crossing very 

 thin films between much thicker ones, or interposing plates at 

 an angle of 45 to the others. 



Mica-selenite combinations after Norremberg, built up of 

 elements consisting of two parallel micas, with a selenite 

 between them either parallel or across, these triple ' elements ' 

 being superposed in various ways and number, crossed or 

 parallel, give in highly- convergent light the most beautiful 

 projections. With no geometrical design whatever from the 

 hand of man (except the crossing of films in various ways), 

 the most exquisite coloured patterns are produced by con- 

 vergent polarised light, some of them looking more like the 

 most brilliant designs in squares of Turkey carpet than any- 

 thing elso. 



223. Polarisation by Small Particles. The blue colour 

 and polarisation produced by all reflection of light from 

 sufficiently small particles (as in the sky), may be easily 

 demonstrated by Tyndall's method, a large glass tube with 



1 For details of such crossed preparations, see my paper in Proceedings 

 of the Physical Society, entitled ' Optical Combinations of Crystalline Films, 1 

 reprinted in Phil. Mag. May 1883. 



