CHAPTER VI. 



CHANNELED SPECTRA OCCURRING IN CONNECTION WITH THE DIFFRAC- 

 TIONS OF REFLECTING GRATINGS. 



44. Introductory. -Throughout the preceding work I had noticed that 

 the spectrum due to either of the component beams, after successive reflection 

 from two reflecting gratings, was often regularly furrowed by transverse 

 black bands, before the two spectra were brought to interfere. As these 

 fringes are stationary, they do not modify the phenomenon investigated ; but 

 questions now arise as to whence these reflected fringes of a single beam come. 

 They are not strong, as a rule, and I was therefore inclined to attribute them 

 to some imperfection of the silvering of the opaque mirrors, but this proved 

 not to be the case, so that it seemed worth while to examine them by special 

 experiments. 



45. Apparatus. The apparatus for this purpose, as one beam only is 

 wanted, is quite simple. In figure 70, L is a vertical blade of parallel rays of 

 white light from a collimator and slit. These rays impinge on the plane grat- 

 ing G, whence the orders 1,2, 



3, etc., of spectra are reflected. && -j % ^' 



Either of these pencils may be 

 received by the second grat- 

 ing G f , plane or concave, from 

 which spectra of any order 

 are available . If o denotes the 

 reflected pencils, the groups 

 from two gratings may be dis- 

 tinguished as (3, i), (3, o), 

 (3. i), (3. -2), etc., as in 

 the figure. Any of these two 

 different pencils is to be examined at T by a lens or telescope, for instance, 

 and the latter (with strengthened objective where needed) is more convenient, 

 even when the concave grating is used. A wide slit S, revolvable about G, 

 is often useful for screening off spectra or parts of spectra. In some experi- 

 ments the grating G' may be replaced by an opaque mirror. 



The gratings are provided with the usual adjustments for parallelism of 

 rulings and slit. G' and T must be capable of considerable right-and-left 

 motion, and G, in particular, of controllable fore-and-aft motion. 



46. Scattering. An interesting result of this work is the evidence and 

 spectroscopic quality of scattered rays, incidentally encountered. For instance 

 in figure 70, if the slit 5 is narrow, it cuts off all the rays but the orange yellow 

 of the third order, and the reflected spectra (3, i), (3, i), etc., will largely 



95 



70 



