slits cut in two parallel sheets of lead, 3 mms. thick. The small spark is placed on one side of the pencil at such a distance that it cannot be reached, even by the penumbra; this is ascertained by proving that the interposition of a sheet of lead causes no diminution of its brightness. Now let us interpose in the pencil an equilateral quartz prism, with refractive edge on the side away from the spark. If the prism is properly set, the spark becomes much more brilliant; when the prism is removed, the spark reverts to its former faintness. This phenomenon is certainly due to refraction, for if the setting of the prism is altered, or if the prism is replaced by a plate of quartz, no effect is observed. The experiment may also be carried out in a different manner: the pencil is first made to impinge directly on the spark, then it is deviated by means of the prism, and the brightness of the spark wanes. If, now, the spark is moved laterally towards the base of the prism, it recovers its previous brightness, proving that the rays in question have been deviated in the same sense as rays of light.
Refraction being thus proved, I at once
