404 



Dr. F. P. Kerschbaum : Interference 



Till now we have assumed that the telephoto camera has 

 to be included in .the vacuum-tube. This is, however, not 

 necessary at all. Just after the light has passed the double 

 slit and before it hits the front of the objective lens, the 

 vacuum-tube can be closed by means of a plane-parallel plate 

 without changing the character of the phenomenon. For it 

 is irrelevant whether the light units fall first, after their 

 passage through the double slit, upon the lens or upon a 

 plane-parallel plate and then upon air of atmospheric pres- 

 sure and the lenses. The extreme vacuum has, therefore, 

 only to be maintained in a tube of about 60 cm. length, 

 containing no lens and no photographic plate. 



The Apparatus. 



I now describe the apparatus which was constructed for 

 the attempt to produce interference- fringes in a highly 

 rarefied gas. 



As the adopted source of light is ultra-violet the whole 

 vacuum vessel has to be made of transparent quartz-glass. 

 Another advantage of this material is that in such quartz- 

 glass vessels an extreme vacuum can be much more easily 

 obtained than in a vessel of any other material. 



The apparatus is shown in the diagrams, figs. 1, 2, 3. 



-FIR'. 1. 



-J- 





i 



■ 62o- 

 /n. 



Charcoal 



Liquidair 



tube. 



Sai^J 



Fig. 2. 



A quartz-glass tube (t) of 600 mm. length, 15 mm. interior 

 diameter, and of 1 mm. wall thickness, has a cylindrical 

 ground surface of 10 mm. length on the outside at (b) (left- 

 hand side in the diagram). Over this ground surface a 



