REVERSED AND NON-REVERSED SPECTRA. 85 



continuous striations on rotating N, but the whole field may easily be filled 

 with nodules. The occurrence of two maxima is probably an incidental 

 result, as in other adjustments but a single one appeared. Naturally the rota- 

 tion of the grating or of the mirrors M and N changes the path-difference of 

 the pencils crossing within it, so that the micrometer screw //, / ^ \\\ 

 at the mirror M must be moved in compensation. Thus /// 

 this is another method of displacement interferometry and y/ f 

 the usual equation suffices. $ ^ v W 



The following rough experiments were made: Placing 

 the strong fringes in the center of the field (slit image), 

 the reading of the micrometer was taken. Then a thick glass plate, = 0.71 

 cm., was inserted in one beam, nearly normally, and the micrometer displace- 

 ment, AN, was found when the fringes were brought back to the center of the 

 field again. The results were (for instance) 



o.375 0.393 cm. 



The displacement equation is (n being the index of refraction of the plate) 



where the correction for dispersion may be put 2B/\ 2 0.026. Hence IJL = 

 1.50, 1.52, as was anticipated. On using white light, where there is but a 

 single strand, a cross-hair, and greater care as to the normality of the plate 

 compensator, etc., there is no reason why results of precision should not be 

 obtained. 



38. The same. The linear phenomenon. The occurrence of the linear 

 phenomenon reciprocally with the fringes for homogeneous light is interesting. 

 It usually appears when there is a flash of the arc lamp, i.e., a displacement 

 of the crater, introducing white light into the sodium arc. It is thus undoubt- 

 edly due to the reversed spectra for white light and may, in fact, be produced 

 by using the white arc or sunlight in place of the sodium arc. When the 

 mirror M is displaced on the micrometer parallel to itself, the linear pattern 

 moves through the wide-slit image from right to left; or the reverse. It does 

 so also when either mirror, M or N, is slightly rotated on a vertical axis. The 

 change in appearance during this transfer is very striking. In the middle, 

 between the extreme right and left positions, the linear phenomenon is excep- 

 tionally strong and fairly tumbling in its mobility. Toward the right or left 

 from the center it becomes gradually less intense, and on one side merges into 

 the homogeneous striations which then appear. On the other side it seems 

 merely to vanish. Doubtless the linear phenomenon is found, as usual, at 

 the line of symmetry of two reversed spectra; but, as both spectra are 

 shrunk to very small lateral dimensions, many colors probably adequately 

 coincide. In an achromatic reproduction of the slit all colors will coincide. 



It is thus not necessary that the edges of the slit images should be super- 

 posed to produce the linear phenomenon. What is still more curious is the 



