Long Waves emitted by the Quartz Mercury Lamp. 693 



quartz- mercury lamp — omitting the radiation filter — showed 

 very irregular character. Nevertheless, it was evident that 

 the main element of the investigated radiation was supplied 

 by a radiation of about the same mean wave-length as that 

 resulting from the Welsbach mantle with this arrangement. 

 But as soon as a 15 mm. thick layer of quartz was inserted, 

 the aspect was changed. The first minimum, which had 

 been observed for unfiltered radiation at a thickness of the 

 air-film of about 5 divisions of the drum* (26 //,), now did 

 not appear before a thickness of the layer of air of 8 divi- 

 sions (42 fju). If the thickness of the inserted layer of quartz 

 was increased to 42 mm., the first minimum appeared only 

 at a distance of the interferometer plates of about 13 divisions 

 (68 a*). At the same time the interference curve showed a 

 much smoother course. The originally observed irregular 

 maxima and minima had nearly quite disappeared ; and, 

 besides the mentioned minimum at 13 divisions, in some 

 series of observations a faintly marked maximum appeared in 



o 10 



the further course of the curve. Such an interferometer- 

 carve is exhibited in the accompanying figure (curve a). 

 Carve (6) of the same figure was observed in the same way 

 with insertion of the 2 mm. thick plate of amorphous quart/:: 

 curve (c) with insertion of the black cardboard (0*4 mm. 

 thick). Curve (c) shows the wave-character most distinctly. 

 Here the minimum lies at 15 divisions (78'4/a) and the 

 * One division of the drum to 5'23ft. 



