THK NEW SPECTRUM. (^85 



energie.s, the familiar prismatic representation enormoasly exao-o-erates 

 the importance of the visi hie, and still more of the ultra-violet recTion, and 

 similarly the grating spectrum exaggerates that of the infra-red reJion. 

 Now he had given, on the map ])efore them, and through the whole infra- 

 red, the exact rock salt i)rismatic spectrum, but for the purpose of 

 obtaining a length which represented (though insufficiently) that of 

 the visible spectrum, he had laid the latter down on the aiumg.' dis- 

 persion 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 way, though it gave the visible energy too small. 



Let 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 colored specti-um, it is 

 not here that the real creative energy of the sun is to be studied, l)ut 

 elsewhere, on the right of the drawing, in the infra-i-ed. 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 

 somewhat insufficientl}^ shown, and on the right extends the great 

 invisible spectrum in which four-fifths of the solar energies an; now 

 known to exist. 



Of this immense invisible region nothing was known until the year 

 1800,^ when Sir Williaiu Herschel found heat there with tiic ther- 

 mometer. 



After that little Avas done" (except an ingenious experhnent by Sir 

 John HerscheP to show that the heat was not continuous) till the 

 first drawing of the energy curve by Lamansky.* in 1871. which, 

 on account of its great importance in the history of the sul)ject, is 

 given on the map. It consists of the energy curves of the visible 

 spectrum, and beyond it, on the right (and in illustration of what has 

 just been said it will be seen how relatively small these latter appear), 

 of three depressions indicating lapses of heat in the infra-red. It is 

 almost impossible to tell what these lapses are meant for, without a 



1 Philosophical Transactions, vol. xc, p. 284, 1800. 



2 It shonld, however, be mentioned that an important paper by Draper (T.ondon, 

 Ed. Dublin Phil. Mag., May, 1843) was published in 1843, in which he appeai-s to 

 claim the discovery of the group here called p6v and which is now known to have 

 a wave-length of less than \^ . (Its true wave-length was not determined tdl nmcli 

 later ) Later, Fizeau seems to have found further irregularities of this lieat a.H long 

 ago as 1847, and of its location, obtaining his wave-lengths by means of interference 

 V)ands. His instrumental processes, though correct in theory, were not exa.-t_ in 

 practice; and vet it seems pretty clear that he obtained some sort^ of recognition 

 of a something indicating heat, as far down as the great region immeduiteiy above D. 

 on our present charts. Mouton (Comptes Rendus, 1879) contirmed this ..bservatioi, 

 of Fizeau's and contrived to get at least an approximate wave-lengtli ut the point 

 where the spectrum (to him) ended, at about 1.8a' . 



» Philosophical Transactions, vol. cxxx, p. 1, 1840. 



*Monatsberichtederk. Akademie der Wissenschaften /.u berlin, Dcvmber, is.l. 



