Dr, A. H. Curtis on Extraordinary Reflection. 



367 



continues true of the rays into which they may be divided sub- 

 sequently by reflection and refraction at a common plane surface, 

 and consequently is true, not only of the rays which reach the 

 eye on emergence from the crystal, with which we are at present 

 concerned, but also of the rays reflected back into the crystal at 

 its upper surface. 



The intensities of the four rays, however, are, as might be ex- 

 pected, in general different ; and in fact, varying the position of 

 the crystal, any one may be made to vanish while the other three 

 continue to exist. The phenomena of crystalline reflection may 

 be exhibited by the apparatus represented in the annexed figure. 



It consists of a horizontal circular stage, B, movable round, and 

 along, a vertical axis, C, passing through its centre. The crystal D 

 (Iceland spar, well polished, for example) is placed on B. The light 

 falls on it through a tube, A, covered with a cap, in which a small 

 orifice is made. E is a tube situated similarly to A but at the 

 opposite side of the crystal, while the height of the stage B is 

 so adjusted that the light which passes through the tube A 

 will, after reflection at the surface of the crystal which is in con- 

 tact with B, and refraction at the opposite surface, pass through 

 the tube E. The tubes A and E may be made movable round hori- 

 zontal axes perpendicular to the plane of the figure, so as to allow 

 of the angle of incidence being altered at will. The apparatus 

 then being adjusted, if the eye be applied to the tube E, five 

 images of the small orifice in the cap of the tube A will be seen 

 — one formed by reflection at the upper surface of the crystal, 

 which requires no consideration, and the other four by the double 

 reflection of each of the refracted rays. As the stage is turned 

 round its vertical axis these images may be four, three, or two — 

 four in general, three when the azimuth of the crystal is such as 

 to cause the intensity of one of the reflected rays to vanish, and 

 two when the incidence and azimuth are such as to give but one 



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