Prof. S. P. Langley on the JSew Spectrum. 121 



by fads, which are independent of our choice, such as the 

 nature of the radiant body or the absorption which the ray 

 has undergone. Bejond this [Nature has no law which must 

 govern us." 



Everything in the linear presentation, then, depends on the 

 scale adopted. In other words, if we have the lengths pro- 

 portionable to the energies, the familiar prismatic representa- 

 tion enormously exaggerates the importance of the visible, 

 and still more of the ultra-violet region, and similarly the 

 grating spectrum exaggerates that of the infra-red region. 

 Now he had given, on the map before them, and through the 

 whole infra-red, the exact rock-salt prismatic spectrum, but 

 for the purpose of obtaining a length which represented 

 (though insufficiently) that ot the visible spectrum, he had 

 laid the latter down on the average dispersion in the infra- 

 red, which was perhaps as fair a plan as could be taken for 

 showing the approximate relation of the two fields of energy 

 in an intelligible wav, though it gave the visible energy too 

 small. 



L t us recall, then, at the risk of iteration, that in spite of 

 the familiar extended photographic spectra of the hundreds 

 of lines shown in the ultra-violet, and in those of the coloured 

 spectrum, it is not here that the real creative energy of the 

 sun is to be studied, but elsewhere, on the right of the draw- 

 ing, in the infra-red. Looking to the spectrum as thus 

 delineated, next to the invisibly small and weak ultra-violet, 

 comes the visible or Newtonian spectrum, which is here some- 

 what insufficiently shown, and on the right extends the great 

 invisible spectrum in which four-fifths of the solar energies 

 are now known to exist. 



Of this immense invisible region nothing was known until 

 the year 1800*, when Sir William Herschel found heat there 

 with the thermometer. 



After that little was done t (except an ingenious experi- 



* Philosophical Transactions, vol. xc. p. 284 (1800). 



t It should, however, be mentioned that an important paper bv Draper 

 (Lond. Ed. & Dublin Phil Mag., May 1843) was published in*1843, in 

 which he appears to claim the discovery of the group here called par and 

 which is now known to have a wave-length of less than 1^. (Its true 

 wave-length was not determined till much later.) Later, Fizeau seems 

 to have found further iire-iilarities of this heat as long ago as 1847, and. 

 of its location, obtaining his wave-lengths by means of interference-lands. 

 His instrumental processes, though correct in theory, were not exact in 

 practice : and yet it seems pretty clear that he obtained some sort of 

 jecognition of a something indicating heat, as far down as the great 

 region immediately above Q on our present charts. Mouton {Cumptes 

 Bendus, 1879) continued this observation of Fizeau's and contrived to get 

 at least an approximate wave-length of the joint where the spectrum (to 

 him) ended, at about !•£/*. 



