S. P. Langley — New Spectrum. 405 



Looking then at the map ; first, in the spectrum on the left 

 and beyond 0^*4 is the ultra-violet region, in fact almost invis- 

 ibly small, but which in most photographs shows over a hun- 

 dred times larger than the whole infrco-red. It really contains 

 much less than one-hundredth part of the total solar energy 

 which exists. Beyond it is the visible spectrum, containing 

 perhaps one-fifth the energy of the infra-red. 



As the writer has elsewhere said, "the amount of energy, 

 in any region of the spectrum, such as that in any color, or 

 between any two specified limits, is a definite quantity, fixed 

 by facts, which are^ independent of our choice, such as the 

 nature of the radiant body or the absorption which the ray has 

 undergone. Beyond this, Mature 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 propor- 

 tionable to the energies, the familiar prismatic representation 

 enormously exaggerates the importance of the visible, and still 

 more of the ultra-violet region, and similarly the grating spec- 

 trum 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 of 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 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 

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

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

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

 next to the invisible ultra-violet comes the visible or New- 

 tonian spectrum, which is here somewhat 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 donef (except an ingenious experiment 



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



f It should, however, be mentioned that an important paper by Draper (London, 

 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 



