Experiments in a Highly Rarefied Gas, 403 



the light through the double slit hit the screen, just as if 

 X-rays were emitted from the line source. 



But Young's simple arrangement is not powerful enough 

 for our purpose. For in order to produce with it distinct 

 interference-fringes, even with monochromatic light, the 

 distance between the source of the light-waves and the screen 

 — or the photographic plate in the case of ultra-violet light — 

 must be several metres. It requires brilliant light or very 

 considerable exposures. Now to maintain a vacuum of at 

 least 0*00001 mm. pressure in such big vessels for a couple 

 of days or even weeks is certainly rather difficult and 

 hopeless if the vacuum apparatus has to be built to hold a 

 photographic plate. 



But Frauenhofer's modification of Young's arrangement 

 helps over this difficulty. The double slit is placed right in 

 front of the objective lens of a telescope or a telephoto 

 camera. I find that by this arrangement the distance 

 between the line source of light and the double slit can be 

 reduced to about 60 cm. when a telephoto camera of only 

 28 cm. length is employed. The telephoto camera enlarges 

 only the interference pattern. The camera of the construc- 

 tion to be described later, produces the narrow interference- 

 fringes which cross the diffraction pattern of the order 

 nought in great intensity. The fringes in the spectra of 

 higher order are, however, much weaker ; they only appear 

 at the plate when the central fringes are already much over- 

 exposed. The distinctness of the interference-fringes does 

 not suffer if the openings in the double slit are made rather 

 wide, between 0'0i and 0'08 mm., and in doing this the gain 

 in intensity is very great. But the first slit representing 

 the line of light must not be wider than 0*02 mm. 



It is evident that for a wave radiation the introduction of 

 a telephoto arrangement does not alter the character of the 

 phenomenon under consideration. 



But for light unit radiation, according to the views given 

 above, a change of theoretical importance will occur. The 

 light units after passing through the double slit fall upon 

 the front of the objective lens, i. e. on matter, and can excite 

 there resonance emission along two lines forming sources of 

 something like a continuously spreading wave. As this sort 

 of wave only comes into existence at the lens, t. e. behind 

 the double slit, and as its course is not limited by any 

 obstacle, no diffraction or interference pattern can be ob- 

 tained on the photographic plate of the telephoto camera. 

 We have, therefore, to expect a patch of uniform illumina- 

 tion on the plate with its centre at the spot where for wave 

 radiation the interference-fringes are most distinct. 



