REVERSED AND NON-REVERSED SPECTRA. 103 



obtain spectrum lines normal to the axis of the spectrum, so that if the latter 

 are superposed the lines will be at an angle. But if this is small, it does not 

 seriously interfere with the occurrence of fringes, as they extend from top 

 to bottom of the spectrum. 



The appearance in general is of the linear character heretofore described. 

 They pass symmetrically from extreme fineness, through a maximum size, to 

 fineness again, with the fore-and-aft motion of the grating G, and they usually 

 rotate near the maximum. 



If the mirror M is displaced nearly in a direction normal to itself, on a 

 micrometer, the fringes undergo the same evolution, and in this respect differ 

 from the case where the primary differentiator, P, was also a grating. In 

 this case the displacement of M showed no discernible modifications of the 

 size or character of the fringe pattern. The fringes merely moved. In figure 

 72 the effect of moving G or M fore and aft is similar, since it throws the 

 point of convergence of the rays NG and MG in front of or behind the grating. 

 The result is therefore different when white light impinges on G from what it 

 is when the light is already nearly homogeneous. 



The limit of visibility is also inferior to the double-grating method heretofore 

 used, for the fringes passed between the limits of visibility through the maxi- 

 mum size, for a displacement of M of only about 3 mm. Smaller ranges 

 may occur. On limiting the incident beam at L to a breadth of about 0.5 cm., 

 the fringes became much broader and relatively intense. 



There is, of course, an abundance of light, so that the screening of the 

 incident beam is not disadvantageous. In this case, when the fore-and-aft 

 position (illuminated strips on the grating coincide, as in figure 72) and the 

 position of the grating relative to its normal axis were carefully adjusted, 

 large arrow-headed fringes, as in figure 73, were obtained, usually less closely 

 packed vertically. Apart from tremors, these move slowly up and down 

 (breathing), as a result, no doubt, of changes of temperature in the air- 

 paths. A mica film inserted into one beam and slowly rotated produced 

 similar motion, besides introducing its own grid of vertical and parallel fringes. 

 The reason for the occurrence of these arrows is not quite clear to me, though 

 they are associated with horizontal fringes and homogeneous light, the doubly 

 inflected forms belonging to inclined fringes and homogeneous light. 



In the endeavor to reproduce these fringes with the sodium arc, I failed 

 after long trials. The reason may be sought in the flicker of the arc, whereby 

 the beam passes from one side to the other of the edge of the prism P, but it 

 is probably due to the inadmissibility of a wide slit. 



53. The same. Crossed rays. The present method, using four mirrors, 

 has, nevertheless, the advantage of admitting the use of either parallel or 

 crossed rays. Inasmuch as these rays are white until they leave the grating, 

 the method is interesting. On being tested it showed the same peculiarities 

 as the preceding. The crossed rays (cc', figure 72) are more nearly normal 

 to the mirrors M and N; nevertheless the range within which the interfer- 



