Experiments in a Highly Rarefied Gas. 



405 



•short brass cylinder fits tightly. The end of this cylinder is 

 -closed by a brass disk bearing the slit (s), which has to act 

 .as the line source of light. This slit was made in the 

 following way: two plane brass plates of 15 mm. length, 



Fig. 3. 



12 mm. height, and 1 mm. thickness, were pressed together 

 with a strip of copper-foil between them, of 0*02 mm. thick- 

 ness. The brass plates were soldered together by means of 

 a small brass band along the top and bottom edge ; after- 

 wards the copper-foil was pulled out. In this way a slit was 

 got of O02 mm. width, 12 mm. height, and 15 mm. length. 



The other end of the quartz-glass tube (t) has a short 

 ground surface on the inside. Into this surface fits a thin 

 conical ring (d) of nickel-steel, 5 mm. long. Across the 

 ring a thin sheet of copper is soldered in which two parallel 

 •openings are cut with a knife, forming the double slit. 

 Each of these cuts is about 12 mm. long, 0*05 mm. wide ; 

 they are 1 mm. apart. Now the ring with this double slit is 

 pressed tightly into the corresponding ground face at the 

 end of the quartz tube. After this the brass tube carrying 

 the slit (s) is put on the front end and turned round till 

 both slit systems are parallel to each other. If the slit (5) 

 is in a symmetrical position to the double slit at the other 

 end of the quartz tube, interference-fringes can be soen when 

 looking through the system, the slit (s) being held towards 

 a source of light and the double slit in front of the eye. 

 Great care has to be taken to find the symmetrical position 

 of the slit systems giving the greatest possible intensity and 

 distinctness of the fringes. The friction in the ground 

 surfaces keeps front and double slit in a fixed position 

 relative to each other. The whole tube {t) with its optical 

 arrangement is then pushed into a quartz-glass tube (n) of 

 20 mm. interior diameter till the ground surface {g) on the 

 inside quartz tube (t) fits into the corresponding ground 

 surface (jf) of the outside tube (n). The other end of this 

 tube is closed with a plane-parallel plate of transparent quartz- 

 glass fused on the tube. The outside tube is just so long- 

 that the inside tube with the double slit reaches right up to 

 the plane-parallel plate if the tube makes a tight fit in the 



