unrecognized Wave-lengths. 153 



feeblest (so that the heat in parts of this region is something 

 like roVo" P ar ^ °f that in the corresponding prismatic spectrum), 

 but instead of one visible, we have now to deal with numerous 

 invisible spectra, overlapping each other. 



Here, then, the slit S 2 has an additional function to fulfil, 

 namely, with the aid of the prism, to discriminate these 

 invisible spectra from each other. Thus in the position 

 actually shown in the drawing, which corresponds to a wave- 

 length of 3^*5341, L e. of 35,341 of Angstrom, we should see 

 the slit covered by a bright spectrum due to several of the 

 higher orders, while we know that energy of the wave-length 

 we are seeking is wholly invisible. If we place a pellet of 

 sodium in our electric arc, we shall see the two sodium-lines 

 on the slit-plate, of which D 2 will fall exactly on the slit, if it 

 be in adjustment, but this sodium-line evidently does not 

 belong to the wave-length we are seeking. 



There are, in fact, passing through the same slit and lying 

 superposed on one another by an unavoidable property of the 

 grating, an infinite number of spectra in theory, of which in 

 this case nearly 20 are actually recognizable by photography, 

 by the eye, or by the bolometer ; arid of which, to consider 

 only those where the wave-length is equal to or greater than 

 that of the sodium-line D 2 *, w © have six spectra as follows : — 



a (visible) 6 th 



spectrum 



. . D 2 



X=0-5890 



b „ 5th 



jj 



• • fl>2 



0-7068 



c (invisible) 4th 



)> 



• f D 2 



0-8835 



d „ 3rd 



>i 



■ • f D, 



1-1780 



e „ 2nd 



)) 



. f D 2 



1-7670 



/ „ 1st 



?» 



. 6D 2 



3-5341 



It is in this invisible underlying 1st spectrum, buried, so to 

 speak, beneath five others, of which three are themselves in- 

 visible also, that lies the wave-length we are seeking ; conse- 

 quently there are (to consider no others) at least six qualities 

 of heat, of six distinct refrangibilities, whose wave-lengths are 

 equal to or greater than that of D 2 , which pass simultaneously 

 through the slit S 2 . They pass through the prism, and on looking 

 through a telescope occupying the position of the bolometer- 

 tube, we shall, by suitably directing the arm of the spectroscope, 

 see the light from the sixth one at a. Its wave-length will be 

 0^*5890, corresponding to a measured deviation (in the case 

 of the rock-salt prism of an angle of 60° 00' 00" and a tem- 



o 



* We have heretofore adopted Angstrom's notation in calling the more- 

 refrangible sodium-line " D } . We shall hereafter, however, in conformity 

 with the now more general usage, call this line, whose wave-length in 



o , o 



Angstrom is 5889, " D 2 ". The corrections to Angstrom are due to the 

 researches of Messrs. Peirce and Rowland. 



