CHAPTER III. 



THE INTERFERENCES OF THE NON-REVERSED SPECTRA OF TWO GRATINGS, 

 TOGETHER WITH AN INTERPRETATION OF THE PHENOMENA IN 

 CHAPTERS I AND H. 



19. Introduction. Method. The chief purpose of the present paper is 

 the search for phenomena similar to those of Chapter II, but in which the 

 two spectra brought to interference are not inverted relatively to each other. 

 Incidentally the strong interferences may have a value on their own account. 

 It has been shown that the totality of the phenomena with spectra reversed 

 on a transverse or a longitudinal axis are quite complicated, and a series of 

 companion researches in which similar results are aimed at, in the absence 

 of inversion, is thus very desirable. 



The apparatus (fig. 30) is a modification of that shown in figure 50, in the 

 next section, MM being the base of the Fraunhofer micrometer, 55 the slide, 

 E the micrometer screw. The brass capsules A and B are securely mounted 

 on the slide 5, free from the base M, and on the base M free from the slide 



_ 



6 



-&> 



c/M, 



J' ^ 31 



30 



5, respectively. Each capsule is provided with three adjustment screws 

 relative to horizontal and vertical axes a, b, b r , and c,d,d r > together with strong 

 rearward-acting springs, by which the gratings G and H at a distance e apart 

 may each be rotated slightly around a vertical or a horizontal axis (plane dot 

 slot mechanism) . The two gratings G and H must be identical, or very nearly 

 so, as to the number of lines per inch, and with their ruled faces toward each 

 other. These faces, as well as the ruled lines, are to be nearly in parallel. 

 To secure the latter adjustment a bolt, g, normal to the face of the grating 

 H, serves as an axis, and an available tangent screw and spring (not shown) 

 is at hand for fine adjustment. This device is of great importance in bringing 

 the longitudinal axes of the two spectra due to G and H into coincidence, 

 and a fine wire must be drawn across the slit of the collimator to serve as a 

 guiding-line through the spectrum. Any lack of parallelism in slit and rulings 

 rotates the fringes. 

 46 



