REVERSED AND NON-REVERSED SPECTRA. 61 



more dispersed than the other, owing to the difference of angles of incidence 

 i, i' and to the residual or differential dispersion at the calc-spar prism, for 

 only the a pencil has been adequately achromatized. In my apparatus the 

 distances CM and GM were about 130 and 100 cm. and the distance NM 

 roughly 30 cm. N is on a micrometer, as it is nearer to the observer at T, 

 though it would be preferable to put M on a micrometer. 



To obtain interferences of the present kind, the plane of polarization of 

 either a or a' must be rotated 90. This may be done by two quarter-wave- 

 length micas M, in a', the first of which (set at 45 as usual) produces circu- 

 larly polarized light and the second then erects the vibration into parallelism 

 with the vibration of the pencil a. Other methods will presently be resorted to. 



With this adjustment fringes were found at T after some searching. They 

 were large, but occurred in a transverse strip of spectrum about DiD 2 /2m 

 width. True, the spectra are without reversion; but, as stated before, one 

 was about one-fifth longer than the other. This is probably the reason for 

 the narrowness of the strip, for the fringes should otherwise fill the spectrum. 



The fringes as first found admitted a displacement of N of about 0.3 cm. 

 only. They were hard to control, needed sharp longitudinal adjustment, and 

 when lost were difficult to find again. They rotate from and to vertical hair- 

 lines, through a horizontal maximum. They are always found in the line of 

 symmetry, which for unequal spectra moves more rapidly than the Fraun- 

 hofer lines if they are separated. The nearer quarter-undulation plate at 

 W may be considerably rotated without quite destroying the fringes. 



If the double quarter-wave-length plate is suitably put in the a beam, 

 results of the same kind are naturally obtained. If a single quarter- wave- 

 length plate is placed in each beam, at 45, both will be circularly polarized, 

 the position of the micrometer being intermediate. I tested this case at some 

 length, but found no interferences, as was to be expected. Circularly polar- 

 ized rays of the same sense do not interfere. 



In place of the achromatized calc-spar prism, a double-image Wollaston 

 prism or a double-image Fresnel prism (rotary polarization) could be used. 

 Unfortunately I had neither of these, but the latter in connection with an 

 analyzing nicol would have been worth testing. 



