412 Dr. F. P. Kerschbaum : Interference 



reach the double slit. The other 84 per cent, pass through 

 the resonance chamber (R) and chamber (V) withont being 

 stopped. 



This layer of saturated Hg-vapour of 21 mm. thickness 

 represents the limit to which I could proceed in reducing 

 the density of the matter acting as source of light and filling 

 the interference apparatus at the same time. One might 

 imagine that these conditions eould be improved very much 

 by using the resonance radiation of Hg at a lower temperature 

 still, i. e. by keeping the drop which supplies the vapour at 

 a lower temperature. This, however, is hardly possible. 

 AVhile the intensity of the resonance radiation would be 

 greatly reduced in this way, at the same time the intensity 

 of the exciting light and therefore the amount of stray 

 light would have to be unaltered. But if resonance lio-ht 

 and stray light have intensities of the same order of 

 magnitude, the experiments will not prove anything. 



(IV.) The critical experiment is performed now in the 

 following way : — 



The same arrangement is used as in experiment (II.) and 

 described fully p. 410, but every care is taken to reduce the 

 partial pressure of the air so much, that the traces of gas 

 still left in the apparatus are absolutely to be neglected. 



The apparatus (comp. fig. 1) is evacuated with the Gaede 

 pump till the McLeod shows a pressure of about 0'00002 mm. 

 During the working of the pump the whole quartz apparatus 

 (with exception of the cap carrying the drop of Hg and the 

 mercury-sealing-wax seal) is heated up with the blowpipe 

 and the glass charcoal vessel warmed as high as possible and 

 kept warm. Now the KMn0 4 vessel is gently heated till 2 

 fills the apparatus to a few mm. pressure. The 2 is pumped 

 off, and this washing with 2 repeated again. After this- 

 washing, while all the absorption vessels are hot still, the 

 McLeod is showing a pressure of O'OOOOl mm. and the 

 Gaede pump is working, the quartz apparatus with the two 

 glass charcoal vessels is sealed off at s (fig. 3) from the rest 

 of the apparatus, L e. the 2 -developer, the McLeod and the 

 Gaede pump. Then the absorption- tube (3) is allowed to 

 cool down and is submerged in liquid air. After two hours 

 this absorption-tube is sealed off at s 2 . The absorption- 

 tube (2) is now allowed to cool down and then submerged in 

 liquid air. In this state the apparatus is kept for four hours. 

 Then the quartz tube (q) is sealed off at s 3 with a coal-gas- 

 oxvoen blowpipe. The production of the vacuum in this 

 way takes two days. As the apparatus was sealed off from 



