REVERSED AND NON-REVERSED SPECTRA. 75 



were found without much difficulty and showed a range of displacement (y) 

 of the prism P, right or left, in various adjustments, of at least ^=0.71, 0.6 1, 

 0.67 cm. larger, therefore, than with the forward prism, as was inferred. Of 

 course, much depends upon how far the extremely fine fringes at the begin- 

 ning and end are pursued. 



This method is not very convenient for the present purposes. In the first 

 place, the distance GMPF is given, an unnecessary restriction on the adjust- 

 ment, unless the lens T is replaced by a short-distance telescope, which has 

 other disadvantages. In the second place, the mirrors M and N can not be 

 used for displacement, as they move the Fraunhofer lines of the correspond- 

 ing spectrum. In fact, M or N affords a method for the adjustment of 

 these lines to coincidence. Finally, the amount of overlapping which is 

 usually secured is always very partial, and if the edge of the prism P is not 

 quite sharp and well silvered, the edges are ragged. Even in the right-and- 

 left displacement (y) of P, the spectra are carried bodily with it in front of 

 T. Although this is no serious objection, it is an unnecessary complication. 

 In spite of the brilliant spectra and large ranges, I did not spend much time 

 in developing the method. 



38. Conclusion. General methods. The origin of the phenomenon of 

 reversed spectra seems to be the slit of the collimator, the diffraction of which 

 furnishes a patch of light, effectively i or 2 mm. in breadth, out of which the 

 component rays are to be separated. Spectroscopically the slit furnishes the 

 degree of homogeneous light within which the phenomenon may be developed. 



In the case of inverted spectra, the slit is not primarily necessary, for here 

 the interferences occur in the direction of (or the fringes lie normally to) 

 the Fraunhofer lines and therefore virtually in homogeneous light. The fringes 

 are due to the continuous changes of the obliquity of rays in each separate 

 color and thus belong to the phenomenon of a wide slit with homogeneous 

 light. The fringes of reversed spectra owe their occurrence to the continuous 

 change of the obliquity of rays produced by dispersion and require a spectro- 

 scopic slit. In the case of combined inversion and reversion, the locus of fringes 

 is not far from circular, though the major axis of the ellipse corresponds to 

 the inversion. Obliquity and dispersion are thus about equally effective. 



The patch of light rays of identical origin may now be separated into two 

 component rays by a variety of methods. The white pencil may simply be 

 cleaved by the edge of a sharp silvered prism, or the pencil may be refracted 

 into two beams at a blunt-edged prism, or the separation may be produced 

 by the diffraction of a grating, by polarization, etc. 



Two entirely distinct pencils are thus obtained, subject to independent 

 control, by which the phenomenon of diffraction may be generalized. In 

 other words, in the classical experiments in diffraction, the diffracting system 

 is rigid. Take, for instance, the following experiment, which has a close 

 bearing on the phenomenon of this paper. In figure 52, L is a distant slit or 

 a fine Nernst filament, P the principal plane of the objective of a telescope 



