378 CARNEGIE INSTITUTION OF WASHINGTON. 



It will be convenient to refer to the cases in which one of the two 

 coincident spectra, from the same source, is rotated 180° with refer- 

 ence to the other, on a transverse axis {i. e., an axis parallel to the 

 Fraunhofer lines), imder the term "reversed spectra"; while the term 

 ''inverted spectra" is at hand for those cases in which one of the 

 paired spectra is turned 180° relative to the other on a longitudinal 

 axis (i. e.j an axis parallel to the length of the spectrum). In the 

 book in question^ the latter are merely touched upon, but they are 

 now being investigated in detail and give promise of many interesting 

 developments. A full account is given of what may be seen with a 

 single grating, the linear phenomenon, as it is called, and which if it 

 stood alone would be difficult to interpret. 



The interferences of the reversed spectra are therefore treated by 

 the aid of two gratings, in virtue of which a multitude of variations 

 are inevitably introduced. The linear phenomenon becomes cross- 

 hatched. The phenomena of reversed spectra are thus exhibited in 

 a way leading much more smoothly to their identification. Their 

 resemblance to the diffractions of two independent, spectrally sym- 

 metrical half-wave fronts of hke origin and under separate control 

 is marked. The effect of the occurrence of trains of beating light- 

 waves is less apparent. 



Section III contains a comparison of the interferences of reversed 

 and non-reversed spectra, the latter produced in a way quite different 

 from the author's earUer work. Naturally these in their entirety 

 are even more bewilderingly varied, and particularly so when (in 

 section IV) an intermediate reflection of one spectrum is admitted. 

 But with all this the present work is on more familiar ground, as the 

 author has hitherto, in the publications of the Carnegie Institution 

 of Washington, given such investigations particular attention. 



The flexibility of the new methods is well shown in section V, 

 where separated component spectrum beams may with equal facility 

 be made to impinge in parallel or cross each other at any angle, or 

 perhaps both, with the double result visible in the field of the telescope. 

 In case of crossed rays a remarkable phenomenon is shown, in which 

 very small differences in wave-length imply remarkably large differences 

 in rotational phase (virtually resolving power) of the two intersecting 

 groups of interference fringes, due to the wave-lengths, respectively. 



Spectra obtained with two gratings, or at times even with one grating, 

 are often annoy ingly furrowed with large transverse fringes. These are 

 also investigated and referred to diffractions resulting from residual 

 errors in the rulings. Finally, several examples of the new methods 

 of investigation are given, showing the important bearing of the 

 diffraction at the slit of the collimator in all these experiments. The 

 real or virtual cleavage of a field of diffracted rays as an essential 

 preliminary is here put in direct evidence. 



