404 S. P. Langley — New Spectrum. 



It is chiefly this visible part, which has been hitherto the 

 seat of prolonged spectroscopic investigation, from a little 

 beyond the violet, at a wave-length of somewhat less than 0^*4 

 down to the extreme red, which is generally considered to 

 terminate at the almost invisible line A, whose wave-length is 

 0^*76. On the scale of the actual wave-length of light, then, 

 where the unit of measurement (1/) is T qVo °f a millimeter, 

 the length of the visible spectrum is (V'36. 



The undue importance which this visible region has as- 

 sumed, not only in the eyes of the public but in the work of 

 the spectroscopist, is easily intelligible, being due primarily to 

 the evident fact that we all possess, as a gift from nature, 

 a wonderful instrument for noting the sun's energy in this 

 part, and in this part only. 



While then this part alone can be seen by all, yet the idea of 

 its undue importance is also owing to the circumstance that the 

 operation of the ordinary prism gives an immensely extended 

 linear depiction of the really small amount of energy in this 

 visible part. There is also a region beyond the violet, most 

 insignificant in energy and invisible to the eye, and the associa- 

 tion of this linear extension due to the prism, with the accident 

 that the salts of silver used in photography are extraordinarily 

 sensitive to these short wave-length rays, so that they can 

 depict them even through the most extreme enfeeblement of 

 the energy involved in producing them, also makes this part 

 have undue prominence. This action of the prism and of the 

 photograph is local, then, and peculiar to the short wave-lengths ; 

 and owing to it, all but special students of the subject are, as a 

 rule, under a wholly erroneous impression of the relative im- 

 portance of what is visible and what is not. The spectrum 

 has really no positive dimension, being extended at one end or 

 the other according to the use of the prism or grating employed 

 in producing it. Perhaps the only fair measurement for dis- 

 playing a linear representation of the energy would be that 

 of a special scheme, which the writer had proposed, in which 

 the energy is everywhere the same ;* but this presentation 

 is unusual, and would not be generally intelligible without 

 explanation. 



The map before us will be intelligible when it is stated that 

 it is, as to the infra-red, an exact representation of that part of 

 the spectrum given by a rock-salt prism. The visible and 

 ultra-violet spectrum given here is not exact, for the reason that 

 it would take over a hundred feet of map to depict it on the 

 prismatic scale, though this is caused by but a small fraction of 

 the sun's energy; so monstrous is the exaggeration due to the 

 dispersion of the prism. 



*This Journal, III, xxvii, p. 169, 1884. 



