406 Dr. F. P. Kerschbaum : Interference 



ground joint (</), the distances between the double slit and 

 the quartz plate being 1*5 mm. The ground joint (g) holds- 

 the long tubes in a fixed position and at the same time 

 prevents a rapid diffusion of the Hg-vapour from the chamber 

 (R) in front of the slit (s) into the part behind this slit, the 

 chamber (Y). 



The quartz-glass tube (n) has still another ground surface 

 (/) at the very front end. On this fits a sort of cap of 

 transparent quartz-glass with the corresponding ground 

 surface (/). The interior of this cap forms the chamber (R) 

 just mentioned above, in which the resonance radiation of 

 the Hg-vapour has to be produced. The Hg-vapour is sup- 

 plied from a drop of mercury which is introduced in a small 

 side-tube (o) before the apparatus is put together. For the 

 incidence of the radiation of the existing Hg arc-lamp a 

 special window (w) is provided in the cap. It is a short 

 quartz-glass cylinder of 5 mm. length and of 15 mm. diameter 

 with flat bottom, quite free from air-bubbles. This window 

 forms a depression in the cap, as shown in figs. 1 and 2- 

 By this arrangement we get intense illumination just in 

 front of the slit (s). Window (io) and slit (s) have such a 

 position relative to each other that a beam of parallel light 

 passing normally through the window does not hit the front 

 of the slit but passes about 2 mm. away from it. At the 

 outside the cap is carefully blackened, with the exception of 

 the flat window (w) of course. 



By reflexion from the walls of the chamber (R) some light 

 from the exciting arc-lamp can come through the slit (s). But 

 experiments will show that the intensity of this stray light is 

 only a few per cent, of the intensity of the resonance radia- 

 tion. The resonance (and stray) light filling the chamber (R) 

 is stopped from entering the chamber (V) by any path other 

 than the slit (s) by coating the quartz tube (t) with liquid 

 platinum burned into the surface. 



When ready for an experiment the ground surfaces (/) 

 of the cap and tube (n) are warmed with a blowpipe, some 

 sealing-wax* put on the warm joint and the cap gently 

 pressed on the tube. In this way a perfect seal can be 

 obtained. For the critical experiment, however, this seal is- 

 not perfectly free from objection, and in this case another 

 method of sealing was used. The ground surfaces (/') are 

 put together, the apparatus turned vertically and the rim 

 filled with mercury. Joint and mercury are warmed up 

 with the blowpipe and when hot covered with a layer of 



* The white quality with low vapour-pressure and low softening- 

 point being employed. 



