684 THE NEW SPECTRUM. 



possess, Jis n g-ift from nature, a wonderful instrument for noting the 

 sun's energy in tliis part, and in this part only. 



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

 undue importance is also owing to the circumstanee that the opei'ation 

 of the ordinary prism gives an immensidy extended linear depiction of 

 the really small amount of energ}^ in this visible part. There is also 

 a region b(\vond the violet, most insignificant in energy and invisible 

 to the eye, and the association of this linear extension due to the pi'ism, 

 with the accident that the salts of silver used in photography are extraor- 

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

 depict them even through the most extreme enfeel)lement of the energ}' 

 involved in producing them, also makes this part have undue promi- 

 nence. This action of the ])rism and of the ])hotogi'aph is Ux-al. th(>n, 

 and peculiar to the short wave-lengths: and owing to it, all but special 

 students of the subject are, as a rule, und(>r a wholly erroneous impres- 

 sion of the relative importance of what is visible and what is not. 

 The spectrum has really no positi\e dimension, being extended at one 

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

 in producing it. Pei'haps the only fail" nunisurement for displaying 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;* ])ut this presentation is umisual and would ixoj: be generally 

 intelligil)le 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 nearly a hun- 

 dred /k?^ 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 exag- 

 geration due to the dispersion of the prism. 



Looking, then, at the map: First, in the spectvum 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 times larger than 

 the whole infra-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-tifth the solar energ3\ 



As the writer has elsewhere said, "the amount of energ}'^ 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 inde- 

 pendent of our choice, such as the nature of the radiant body or the 

 absorption which the ray has undergone. Be\'ond 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 proportionable to the 



'American Journal of Scienre, III, xxvii, p. 169, 1884. 



