REVERSED AND NON-REVERSED SPECTRA. 



33 



At these extremes the two patches of light on the grating G' may have 

 been separated by several millimeters. The nature of the transformation 

 from arrows to the oblique striations would be well reproduced if equidistant 

 vertical wedges were moved from right to left, or the reverse, behind a 

 vertical slit. 



The distance between G and G' was about 10.2 cm., and between the spots 

 of light on mirror M and gratings G and G', respectively, 22.2 and 12.6 cm. 

 This corresponds to 01 = 20.5 and 02 = 36.2, so that 6= 15.7 and o- = s6.7. 



It is obvious that if the slit of the collimator is displaced right or left, the 

 range of displacement, within which the interferences lie, will have different 

 positions on the micrometer, because the path-differences are changed. A 

 flickering arc may also introduce annoyances. 



The present method has an advantage for ordinary practical purposes, as 

 it does not require a ruled-glass grating at G. 



20 



The surprising success obtained with the film grating at short distances 

 induced me to test similar methods at long distances. Figure 19 is an appa- 

 ratus of this kind, in which L is the white beam incident from a collimator, 

 G and G' are the transmitting gratings, M, N, m, n, pairs of opaque mirrors, 

 T the telescope. The undeviated ray, d, is screened off. The component 

 paths a+b+c, a'-f-fr'-f-c' were each about 4 meters long. The method of 

 adjustment again consisted in bringing the shadow of the thin wire across 

 the slit into the same position of the spectra seen in the telescope when the 

 spectra coincide. For this purpose the adjustment screws for horizontal and 

 vertical axes on M, N, m, n must be actuated together. To facilitate this 

 tiresome work, with the observer at T, long levers brought from m and n, 

 with their ends near his hands, as well as a lever from G' (fore-and-aft motion), 

 were very useful. Since the adjustment screws at M and N are already 

 within reach, it is thus easy to bring any Fraunhofer line to the middle of the 

 field and to make these fields overlap, with the guide-wire central in both. 



The attempt made with sunlight, to find the fringes when both G and G' 

 are film gratings (D = 167 X icr 6 cm.), did not succeed. The light, moreover, 



