2 8 4 D O C U M E N T 2 7 0 A P R I L 1 9 2 6 270. From Emil Rupp Heidelberg, 29 April 1926 Dear Professor, Thank you very much for both of your letters.[1] I have meanwhile tried out the grating experiment in the following way: K = canal ray after leaving the slit. P = cemented-on plane // sheet of glass. L1 = lens 4 cm focal length at a distance of 2.5 cm from K. L2 = lens, f = 40 cm. L1 & L2 are set to accord with a telescope set at ∞. In the image plane of L1 (17.5 cm from L1), there is a grating G upon which an image of the canal ray is formed. A dia- phragm B of 4 mm width lies on the grating. In front of lens L2 there is a green filter F made of glass. I I indicates the interferometer, b b = the slit diaphragms. Fe = tel- escope. I used Hg canal rays as the light source, line 546 mμ, because with that it is eas- iest to obtain interferences at high path differences. The velocity v was at 35,000 volt 2 · 107 cm/sec. Because the interference capacity of this line remains constant at 4 mm behind the slit, as measured earlier (Ann. p. 17),[2] I installed diaphragm B of 4-mm width before the image plane of L1 at grating G, which selects just the proper part of the canal ray. I got for myself 2 very good grids as the grating. A color filter F is necessary to isolate out the green Hg line. I believe that no optical impurities enter with the selected glass filter, unlike with a gelatin filter. The inter- ferometer I I is set for interferences of equal thickness the location of the interferences is therefore a mirror plane & the telescope must be adjusted to this plane. When adjusting to interferences of equal inclination, owing to imperfections in the screw spindle I cannot shift the movable mirror without the interferences immediately disappearing. The slit diaphragms b b (4-mm wide) are unfortunately necessary, despite their undefinability, in order to achieve the required large path differences. Perhaps I could remove them if a lens with a very large relative aper- ture were available to me instead of L1. My findings up to now with this arrangement are these:
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