REVERSED AND NON-REVERSED SPECTRA. 123 



appearance is now curiously like a blunt wedge (fig. 91), with a band at b 

 nearest the eye, and the lines dd extending quite to the rear. This impression 

 is probably an illusion, due to the shading; the lines grow finer and are more 

 crowded toward the bottom and top of the spectrum. The illusion of a 

 reentrant wedge is not possible. 



To use this interference pattern for measurement, the cross-hair is supposed 

 to pass through the region c (fig. 90) symmetrically. Very slight motion of the 

 micrometer mirror M then throws c either to the right or the left of the cross- 

 hair. In this case the lens doublet, C, is attached to the mirror and moves with 

 it, as stated. To obtain the extreme of sensitiveness, the path-difference of NG 

 and GM must be all but zero; i.e., the grating plate G and the lens doublet C 

 (fig. 87) must be all but compensated for equal air-distances by the compen- 

 sator C'. In this case of full compensation, the interference pattern, in the 

 absence of a doublet C, would be enormous and diffuse, seen preferably in the 

 principal plane of the telescope, but useless for measurement. The introduc- 

 tion of a lenticular compensator, balanced by a compensator in GN, transforms 

 the huge pattern into the small interference fringes in question, with the advan- 

 tage that the high mobility of the coarse design has been retained. In other 

 words, an index suitable for adjustment has been found, compatible with 

 extreme sensitiveness. In fact, it is difficult to place the micrometer mirror 

 M so that the region c (fig. 90) is exactly bisected. As the plane in which these 

 interferences are seen most distinctly is i cm. or more anterior to the principal 

 focal plane, the Fraunhofer lines are unfortunately blurred and a cross-hair is 

 needed as a line of reference. 



I may in conclusion refer to a similar series of experiments now in prog- 

 ress, in which the compensators placed in the M and N pencils (fig. 87, 

 C, C'}, instead of being of different shapes as above, are plates of different 

 kinds of glass (crown and flint, for instance). Here the successive differ- 

 ences of dispersive power, from wave-length to wave-length, produce effects 

 closely resembling those discussed, with the advantage that difficulties 

 inherent in the curved system are avoided. 



