Experiments in a Highly Rarefied Gas. 407" 



sealing-wax. After cooling down, the apparatus can be- 

 placed horizontally, the Hg being kept in position by the 

 solid layer of sealing-wax. In this way an intrusion of 

 vapours due to the sealing-wax is entirely prevented. The- 

 diffusion of the Hg- vapour between the ground surfaces (/') 

 does not matter, because the joint (/) leads to the chamber 

 (R) already saturated with Hg-vapour from the drop (o). 



For the exhaustion of the whole vessel, the quartz tube (>/) 

 has a wide side tube (m) which leads to a wide quartz tube- 

 (1 in fig. 3). This tube is filled with charcoal and when 

 submerged in liquid air is supposed to condense the Hg- 

 vapour, coming through the slit (s) . In order to procure ample 

 connexion between the chamber (Y) and this charcoal tube,, 

 a hole (A) is blown into the quartz tube (t) (compare fig. 1) 

 just opposite the side tube (m). This side tube (in) is fused 

 on the tube (n) as near as possible to the ground joint (g). 



The quartz tube (q) connects the quartz charcoal vessel 

 with the rest of the apparatus provided for the exhaustion, 

 which is made of ordinary glass. Quartz tube and ordinary 

 glass tube are cemented together with sealing-wax at (v). 

 As shown in the figure, two more charcoal tubes are pro- 

 vided and a tube containing some KMn0 4 , for washing with 

 2 , a Gaede pump and a MacLeod gauge (300 c.cm.). 



At (V), (s 2 ), (s s ) places are provided where the glass,. 

 resp. quartz tubes can be sealed off. No taps are provided 

 at all. 



During an interference experiment the evacuated quartz 

 apparatus is held horizontally by two solid supports of a big 

 optical bench, which is free from vibrations. On the same 

 optical bench the telephoto camera slides, carried by two 

 solid supports. This telephoto camera (comp. fig. 1) con- 

 sists of a wide brass tube which holds the objective lens l A of 

 10 cm. focal length and 5 cm. diameter. At a distance of 

 130 mm. the lens l 2 is placed, of 2 cm. focal length and' 

 2 cm. diameter. The end of the brass tube carries a support 

 for a photographic plate-holder. The distance between the 

 lens l 2 and the plate (a) is 150 mm. 



The telephoto camera is brought right up to the plane- 

 parallel quartz plate (p) of the vacuum vessel. It was found 

 empirically that such a camera in connexion with the double- 

 slit arrangement, as described above, could produce inter- 

 ference-fringes of suitable dimensions, quite distinct and 

 numerous enough. 



The light of the wave-length X = 2536 A.E., exciting the 

 Hg-resonance radiation, was supplied from an Hg arc-lamp 

 of special construction. The arc of this lamp emits light 



