﻿S. P. Langley — Unrecognized Wave-lengths. 87 



in the new region which we are for the first time exploring. 

 Glass is impermeable to this kind of heat, but with our rock-salt 

 train and with the delicate apparatus previously described, there 

 is little difficulty in discriminating it by the bolometer, where 

 the prism alone is employed, and in mapping the deviation of 

 each spectral ray as shown in Plate III of our paper in this 

 Journal, Jan., 1886. But now that we wish to determine wave- 

 lengths, the conditions are altogether different ; for not only the 

 grating enormously expands this part of the spectrum and has 

 cut the heat down correspondingly, just where that heat is 

 itself feeblest, (so that the heat in parts of this region, is some- 

 thing like y/oo part of that in the corresponding prismatic 

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

 numerous invisible, spectra, overlapping ecah other. 



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

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

 visible spectra from each other. Thus in the position actually 

 shown in the drawing, which corresponds to a wave-length of 



3^-5341, i. e., of 35341* of Angstrom, we should see the slit 

 covered by a bright spectrum, due to several of the higher 

 orders, while we know that the 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 twenty are actually recognizable, by photog- 

 raphy, by the eye, or by the bolometer, and of which, to con- 

 sider only those where the wave length is equal to or greater 

 than that of the sodium line D 3 * we have six spectra as follows : 



a. (visible) 6th spectrum D 2 „._A=0 / "-5890 



b. " 5th " £D 2 0-7068 



c. (invisible) 4th " f D 2 ... 0-8835 



d. " 3d " |D 2 1-1780 



e. " 2d " |D 2 1-7670 



/. " 1st " 6D 2 3-5341 



It is in this invisible underlying first spectrum, buried, so to 

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



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

 ble sodium line " D^' We shall hereafter, however, in conformity with the now 

 more geueral usage, call this line, whose wave-length in Angstrom is 5889, "D 2 ." 

 The corrections to Angstrom are due to the researches of Messrs. Peirce and 

 Kowland. 



