being birefringent. A movement of the handle of the press, however, gives the crystal a pinch, instantly responded to by the colours on the screen, showing the production of double refraction. Another movement of the handle brightens the colours, and a third may strain the crystal beyond its power of resistance, when the crystal flies to pieces.
The Diamond and Rontgen Rays
The diamond is remarkable in another respect. It is extremely transparent to the Rontgen rays, whereas highly refracting glass, used in imitation diamonds, is almost perfectly opaque to the rays. I exposed for a few seconds over a photographic plate to the X-rays the large Delhi diamond of a rose-pink colour weighing 31J carats, a black diamond weighing 23 carats, and a glass imitation of the pink diamond (Fig. 18). On development the impression where the diamond obscured the rays was found to be 107
