REVERSED AND NON-REVERSED SPECTRA. 55 



slit, these sodium fringes often appear at the same time as the colored fringes, 

 and, though they are usually of different sizes, their lateral displacement 

 with a change of distance apart of the gratings, 8e, is the same. The fringes 

 in question appear alone when the sodium burner is used. They may then 

 (at times) be observed with the naked eye, with or without a lens, and they 

 fail to appear in the telescope unless the objective is strengthened by an 

 additional lens. They are always vertical, but finer in proportion as the 

 DiD 2 and D\D'z doublets are moved farther apart. They become infinite 

 in size, but still strong, when the doublets all but coincide, showing a ten- 

 dency to become sinuous or possibly horizontal. Rotation of either grating 

 G around an axis normal to itself and relative to the other produces greatly 

 enhanced rotation of the fringes, as in all the above cases, but they soon 

 become blurred. 



Only in the case when the horizontal axes of the field coincide (parallel 

 rulings, etc.) do they appear strong. When the angle of incidence (or non- 

 coincidence) is increased for both gratings, the size of the fringes increases; 

 but when the e distance is increased by the micrometer, the fringes are appar- 

 ently constant as to size. However, after displacement of 4 mm. they are 

 liable to become irregular and stringy, though still moving. A fine slit is not 

 essential, particularly when e is small. They vanish gradually when the slit 

 is too wide. If a telescope with a strong objective is used, these fringes may 

 be seen, retaining their constant size long after those of the next paragraph 

 vanish. Examples of data are given in table 6, and 5e is too low in value as 

 compared with the computed datum for 4 = 0. With the Wallace gratings, 

 these fringes were best produced by the aid of the sodium lines, in the ordi- 

 nary electric arc, simultaneously with the colored fringes and for the case of 

 a very fine slit. They were apparent both with an ocular drawn out or drawn 

 in. In the former case several successive groups were observed. Beginning 

 with the sharp sodium lines in principal focus (D 2 and D' 2 coincident) , a slight 

 displacement of the ocular outward showed the first group, this resembling a 

 grid of very fine striations. Further displacement outward produced a second 

 set, equally clear but larger. A third displacement of the ocular outward 

 showed the third set, and these now coincided with, and moved at, the same 

 rate as the colored fringes in the same field. Other groups could not be found. 

 No doubt, for these four successive steps the interference grids of DI and D\, 

 D 2 and D' 2 are coincident and superposed, until they finally find their place in 

 the colored phenomenon. 



TABLE 6. Ives grating. Homogeneous light. Fine slit. Sodium lines not coincident. 



5eXio 3 =o.87 cm. 

 77 

 .83 



23. Homogeneous light. Slit and collimator removed. Fringes similar 

 to those seen with the wide slit above may be observed to better advantage 

 by removing the slit altogether. The sodium flame is then visible as a whole ; 

 and if the adjustments are perfected it is intersected with strong, vertical black 



