S. P. Langley — Observations' on Invisible Heat-Spectra. 3 



and rays whose wave-length is as little as 0185/* have, it is 

 said, been observed from the induction spark. 



Our atmosphere cuts off the ultra violet rays of a length less 

 than about 0*29 fi, while I have found it not very difficult to 

 see, below Fraunhofer's great A, lines whose wave-length is about 

 0*81 fjt. The extreme range of the normal eye then, is from 

 about 0-00036 to 0-00081 mm , or a little over one octave, though 

 the statement that the range of the eye is less than one octave 

 is still commonly made. 



Fraunhofer's first measures were made with a literal grating 

 composed of parallel strands of wire, while the successive labors 

 of Nobert, Eutherfurd and Rowland have placed in the hands 

 of physicists instruments of constantly increasing power, which 

 have finally reached what seems nearly theoretical perfection at 

 the hands of the two latter. It is with the now so well known 

 gratings of Professor Rowland that the direct measures of wave- 

 lengths in the solar heat spectrum I have already made public* 

 have been chiefly executed. 



In Plate I we have a necessarily condensed representation of 

 the whole spectrum, visible and invisible, on the normal scale, 

 the distances being proportional to the wave lengths observed. 

 The inferior limit being 0, we have at (a) the number 0*18 ft 

 (eighteen one-hundred thousandths of a millimeter), which 

 represents the shortest measured in the electric spark from 

 aluminum. Next near 0"29 /j. (b) we have, according to M. 

 Cornu, the shortest solar ray which penetrates our atmosphere; 

 near 0*35 p. (c), in the ultra violet, is the shortest wave which 

 can be seen by the naked eye, and nearly the shortest which 

 can pass through glass, while near 0*81 p. (d) in the extreme 

 red, is nearly the longest which the eye can observe. The 

 entire visible spectrum on the normal scale is, it will be seen, 

 insignificant in comparison with that great infra-red region 

 which is so important to us, and of which we know so very 

 little. It has been known since the time of the first Herschel 

 that heat rays existed below the range of vision, but of their 

 wave-lengths nearly nothing has, till lately, been ascertained, 

 partly for want of sufficiently delicate heat-recognizing apparatus, 

 and still more from the fact that it is difficult to use'the grating 

 here, owing to the overlapping spectra, and to the consequent 

 necessity we have till lately been under, of separating these rays 

 only by the prism, which gives no measure of their wave- 

 lengths. Physicists have accordingly attempted to find these, 

 by observing what deviations correspond to known wave- 

 lengths in the visible portion, and by trying to determi e, 

 from theoretical considerations, what relations should obtain in 



* Comptes Rendus, Sept. 11, 1882. National Academy of Science, 1883. This 

 Journal, March, 1884. 



