NEW SPECIES OF LIGHT g 



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 pheno- 

 menon 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 ob- 

 served. 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 de- 

 viated by means of the prism, and the bright- 

 ness of the spark wanes. If, now, the spark is 

 moved laterally towards the base of the prism, 

 it recovers its preyious 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 



