14 "AT" RAYS 



gases, which are so arranged that no light 

 escapes ; a rectangular orifice, 4 cms. wide and 

 6*5 cms. high, cut in the iron at the same 

 height as the incandescent mantle, is closed by a 

 sheet of aluminium i mm. thick. The chimney 

 of the Auer burner is of sheet-iron, and a slit 

 2 mms. wide and 3*5 cms. high is cut in it, op- 

 posite the mantle, so that the emerging lumin- 

 ous pencil is directed on the aluminium sheet. 

 Outside the lantern, and in front of this sheet, 

 a double-convex quartz lens is placed, having 

 12 cms. focal length for yellow light, behind 

 which is a spark-gap of the kind already de- 

 scribed, giving very small sparks. The spark 

 is produced by a small induction-coil, provided 

 with a rotating make and break device, which 

 works with perfect regularity. 



The distance / of the lens from the slit being 

 26*5 cms., one notes, by help of the spark, the 

 existence of a focus of very great sharpness at a 

 distance, /', of about 1 3 '9 cms. For at this point 

 the spark exhibits a notably greater glow than 

 at the neighbouring points, whether in front or 

 behind, above or below, to the right or to the left. 

 The distance of this focus from the lens can be 



