REVERSED AND NON-REVERSED SPECTRA. 97 



Combination G2, G 3 : Fine set just before the surfaces appeared, which 

 were delicately striated; fine regular set; coarse set, both close to surface; 

 (with lens on) fine regular set; doubled, strong regular set. 



Different distances between G and G' had very little influence on the size 

 of the phenomena. A few examples may be given, which are observed when 

 the ocular is moved outward. 



3, Gi : Distance 10 cm. Fringes, faint regular; strong irregular; faint 

 regular; flat field; surfaces visible; faint regular. 



Distance 25 cm. Strong irregular; faint regular; small regular; large 

 (double) irregular; lines slit into fine fringes; large faint regular. 



Distance 46 cm. Large strong, with two absorption bands; fine regular; 

 double-sized faint ; surfaces with fine striations ; alternations of fine and coarse 

 lines; faint, regular, large, etc. 



Fringes of different color are often in different focal planes. When a lens 

 is used with the concave grating, observations must sometimes be made 2 

 meters off to get the large regular fringes. Red fringes may be narrower than 

 the corresponding violet set. 



If the grating G is moved fore and aft, parallel to itself, the fringes are 

 shifted across the stationary sodium line, as in displacement interferometry. 



Whereas in the positive combination (3, i), (3, 2), etc., the spectra widen, 

 they tend to close up for the negative combinations (3, i), (3, --2), etc. 

 With two identical plate gratings they may image the white slit. But this 

 seems to have little effect on the fringes seen as a whole when the ocular is 

 out of focus. 



When white light is used and the grating G' replaced by an opaque mirror, 

 or in case of combinations which involve direct reflection (2, o; 3, o; etc.) at 

 G', there seem to be no fringes. 



48. Fringes with sodium light. While there is some difficulty in obtaining 

 the fringes with white light, fringes with homogeneous light are obtained at 

 once, provided the light is sufficiently intense. A sodium arc lamp, or a 

 mercury lamp, with a fine slit, must therefore be used. In this case, moreover, 

 the grating G' may often be replaced by an opaque mirror, or the fringes of 

 the order 26*0, GzGo, etc., may be produced with entire success. On moving 

 G fore and aft, they again travel across the sodium line. Often, in fact, two 

 sets of fringes seem to be shifted. A few examples again may be given of 

 the great variety in this display while the ocular is being drawn out : 



Gi,G' 2: Sodium lines DiD z single size ; large strong fringes, lines split. 

 Gi, G'i: Closed spectrum; striations continuous. 

 Gi, G'o: Reflection; DiD 2 single size; surfaces of gratings finely 

 striated. 



Gi, G'I 



Gi, G'2 



G 2 , G'o 



DiD 2 double size; strong grid seen very near the surface of G'. 

 D\Dz treble size, out of reach. 

 Reflection; no fringes. 



