96 THE INTERFEROMETRY OF 



consist of orange-yellow light. Associated with each of these reddish-yellow 

 patches, however, are vividly violet-blue patches, each separated from the 

 reddish yellow by an almost total absence of green, relatively speaking. If 

 the light is very intense, the connecting part of the spectrum also appears, 

 but it is always far less vivid than the ends of the spectrum in question. 



Inasmuch as all violet radiation proper has been screened off at S, it is 

 obvious that violet light must have been scattered in all directions from G, 

 a part of which, therefore, passes the slit and is resolved by the second grating 

 G'. Moreover, as the scattering lines of the grating are equidistant, the 

 scattered light has a regular wave-front. (Cf. Cam. Inst. Wash. Pub. No. 

 229, 1915, pp. 100 102.) 



The correlative experiment of detecting the reddish light transmitted after 

 scattering was also tested. For this purpose the reflecting grating G may be 

 replaced by a transmitting grating, slit 5 placed beyond, and the light then 

 analyzed by a second grating G' behind the slit and diffracting toward it on 

 one side. But no results of value were obtained. 



47. Fringes with white light. The experiments with the apparatus (fig. 70) 

 were commenced with sunlight and (what is essential) a fine slit. Fringes 

 are found in all combinations of doubly diffracted pencils (3, +i), (3, i), 

 (3, 2), etc.; (2, i), (2, i), etc.; (i, i), (i, i), etc., but none in the 

 reflected pencils (3, o), (2, o), (i, o), etc., as a rule. Whether the grating G' 

 be concave or plane, it is best to use a telescope at T, because (when provided 

 at the objective with an auxiliary concave or a convex lens) it more easily 

 offers a wide range of observation along its axis than an ocular. The latter 

 must be wide and has to be shifted bodily; but both methods were used. A 

 concave grating at G and plane grating at G' gave no results. The concave 

 grating is usually more free from channeled spectra. 



Of the great variety of fringes obtained, I shall give only two typical 

 examples. The second order of spectra for G (plane) was separated from the 

 others by the slit S and diffracted into G' (fig. 70). The successive fringes 

 appear as the ocular is drawn outward from the principal focus. 



Combination 2, G'o : Only a good sodium doublet, which became washed 

 on drawing out the ocular of T, was obtained ; no fringes appeared. 



Combination Gz, G'--i: Just outward from the principal focus a large, 

 coarse, irregular set of fringes appeared; next (ocular farther out) a large 

 regular set, somewhat diffuse, possibly double and superposed; then a finer, 

 half-size, very regular set, possibly decreasing. After this the mottled sur- 

 faces of the gratings were successively in focus. A weak spectacle lens was 

 now added to the objective of T, whereupon very large regular fringes were 

 seen when the ocular was far out. 



Combination 2, G 2: The ocular moving outward from the principal 

 focus, the fringes seen in succession were as follows: large, regular, vague; 

 half -size sharp; surfaces vertically striated; (lens on) fine regular set in red; 

 doubled regular set in green. 



