120 Prof. S. P. Langley'oft the JSeiv Spectrum. 



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 

 association 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 importance of what is visible and 

 ■tthat 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. Perhnps 

 the only fair measurement tor displaying a linear representa- 

 tion of the energy would be tbat of a special scheme, which 

 the writer had proposed, in which the energy is everywhere 

 the same * ; but this presentation is unusual and Mould not 

 be generally intelligible without explanation. 



The map before us (PL III.) 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 nearly 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 ex- 

 aggeration due to the dispersion of the prism. 



Looking, then, at the map : First, in the spectrum on the 

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

 invisibly small, but which in most photographs shows almost 

 a hundred tunes larger than the ivhole infra-red. It really 

 contains much less than one hundredth part of the total solar 

 energy which exists. Beyond it is the visible spectrum, con- 

 taining perhaps one fifth of the solar energy. 



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

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

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



* American Journal of Science [3] xxvii. p. 169 (1884). 



