C. Bar us — Interference of Reversed Spectra. 425 



even removing it altogether. A new type of interferences 

 appeared, linear and parallel in character and intersecting the 

 whole yellow field. These lines could (as above) be made to 

 pass from a grid of very fine hair-like nearly horizontal lines to 

 relatively broad vertical lines, on changing the orientation of 

 the grating G', figure 5 or 8. Small changes of position of the 

 gratiug produced a relatively large rotation and enlargement 

 of the lines of the interference pattern. The fringes when 

 vertical and large are specially interesting. The distances 

 between successive fringes obtained were about the same 

 (accidentally) as the D X D„ distance of the sodium lines. They 

 are quiet in the absence of tremor. If D X D' or D^DJ were 

 only present, the field would be an alternation of yellow and 

 black striations. The fringes are nevertheless quite distinct, 

 but a single homogeneous line (like the green mercury line) 

 would give the best results. It is necessary that the line 

 selected (say D X D^) should coincide horizontally and vertically 

 before the slit is broadened. Otherwise no fringes appear in 

 the yellow ground or at least not in the principal focal plane. 

 On using a thin mica compensator, it is easy to control these 

 fringes while the mica film is rotated. The fringes remain 

 identical in size, from their inception till they vanish, while the 

 micrometer M, figure 5, passes (as above) over about 15,000 

 wave lengths. In this respect the new interferometer differs 

 from all other types, the actual lengths of the two air paths 

 G3IG', GJVG' alone being in question. 



7. Experiments continued. — As these fringes were pro- 

 duced with a concave reflecting grating, the question may be 

 put whether they would also appear in case of the plane 

 reflecting grating, G', in the adjustment of figure 5. The ex- 

 periment was therefore repeated with a wide slit, or with no 

 slit at all, and there was no essential difference in the two 

 classes of results. 



On the contrary, when the method of but one grating and 

 sodium light was used (figure 1) the interferometer fringes, in 

 case of a very wide slit or the absence of a slit, could not be 

 produced over the yellow field, as a whole. There appeared, 

 however, an obvious flicker in parts of the field, on reducing 

 the width of the slit till the sodium lines were each about the 

 width of a D X D^ space, with either D^DJ or D^D^ superposed. 

 The sharply outlined slit showed an irregular rhythmic bright- 

 ening and darkening over certain parts of its length. These 

 broad pulsations were very violent, very much in character 

 with the linear phenomenon above. 



Regarding the phenomenon as a whole, one may argue that 

 in case of the wide slit and single grating, in which the lines 

 for both diffractions are therefore rigorously parallel, the in- 



