MEMOIR VI.] 



THE DIFFRACTION SPECTRUM. 



113 



of a grating. The reflected beam went out through the 

 slit at which it entered, and on either side of it to the 

 right and left the well-known double series of diffraction 

 spectra made their appearance. I selected, for the obvi- 

 ous reason that it was not overlapped by its successor, 

 the first of the series, and intercepting it by an achro- 

 matic object-glass, placed in the focus a frame capable 

 of holding a ground -glass or sensitive surface. This 

 frame was adjusted until the fixed lines were distinctly 

 depicted upon it. 



For a further description of the reflected diffraction 

 spectrum I may refer to any of the elementary works on 

 optics. It is sufficient for my purpose here to recall that 

 the angular deviations of any two colors from the primi- 

 tive incident ray are to one another as the lengths of 

 their respective undulations. 



On the ground-glass we see the fixed lines, and the 

 length of waves corresponding to those lines has been 

 carefully determined by Fraunhofer. The following 

 table is extracted from Herschel's treatise on Light, the 

 Paris inch being divided into one hundred millions of 

 equal parts: 



Length of wave corresponding to the fixed line B, 2541 parts. 



C, 2422 



D, 2175 



E, 1945 



F, 1794 



G, 1587 

 H, 1464 



When, therefore, we have any chemical, luminous, ther- 

 mic, phosphorogenic, or any other effect under discussion, 

 in relation to the spectrum, we have only to determine 

 its place among the fixed lines, remembering in the dif- 

 fraction spectrum the simple law that connects the devi- 

 ations and wave-lengths. In the mode of operation here 

 described absolute exactitude is not reached because our 



H 



