REVERSED AND NON-REVERSED SPECTRA. 



27 



(on a micrometer slide) and N, thence to be reflected to the reflecting grating 

 G', plane or curved. Here the two beams of the identically colored light 

 selected are again diffracted to the telescope or lens at T. Since the gratings 

 G, G', rarely have the same grating constant, their proper position must be 

 found by computation and trial. In my work the distances to the line of 

 mirrors NM were 165 cm. for G and 90 cm. for G'. This method automati- 

 cally excludes the direct beam a and all glare, and gives excellent spectra both 

 in the first and second orders. The use of two gratings, however, introduces 

 the difficulties of adjustment specified, as the two D doublets corresponding 

 to N and M will not, as a rule, be parallel and normal to the longitudinal 

 axes of the spectrum, unless all cardinal features, like the rulings and their 

 planes, are quite parallel. If the grating is not normal to the impinging 

 beam, the axis of the corresponding spectrum is a curved line. The spectra 

 are, moreover, likely to be unequally intense, a condition not infrequent 

 even in the preceding method. It is possible that this may be due to the 

 grating itself, but probably unequal parts of the corresponding beams are 



used in the two cases, or the mirrors are unequally good. As a result, in my 

 earlier work I was not able to produce the phenomena with two gratings, 

 after many trials, in spite of the clearness of the overlapping spectra; but 

 the same serious difficulties are encountered whenever interferences are 

 produced from two independent surfaces. 



Later, having added a number of improvements to facilitate adjustments, 

 I returned to the search again and eventually succeeded. There are essen- 

 tially four operations here in question, supposing the grating G approximately 

 in adjustment. By aid of the three adjustment screws on each of the mirrors 

 M and N, figure 16, the fine wire drawn across the slit may be focussed on the 

 grating, if an extra lens is added to the collimator and the black horizontal 

 shadows of that wire, across the corresponding spectra, placed in coincidence. 

 The grating G' is then to be moved slowly fore and aft, normal to itself, on 

 the slide, so that the position in which the sodium lines are nearly in coinci- 

 dence to an eye placed at the telescope, T, may be found. The grating G' 

 is next to be slowly rotated on a line (parallel to LT) normal to its surface, 

 to the effect that the black axes of both spectra (i.e., the spectra as a whole) 

 may coincide. This must be done accurately, and the last small adjustments 

 may be made at the screws controlling M and N. Finally, the micrometer 



