396 Prof. S. P. Langley on Invisible Heat-Spectra 



tively — numbers nearly corresponding with the lines H and 

 B, while Fraunhofer's own values are comprised between 

 0-00036 millim. and 0*00075 millim. More recently the 

 range of vision has been still more extended, by the use of 

 the fluorescent eyepiece of Soret ; while by the aid of photo- 

 graphy and the employment of quartz trains, solar radiations 

 of a wave-length of about 0*29 fi have been observed *, and 

 rays whose wave-length is as little as 0*185 jju 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 p, while I have found it not very 

 difficult to see below Fraunhofer's great A, lines whose wave- 

 length is about 0*81 /x. The extreme range of the normal 

 eye, then, is from about 0*00036 to 0*00081 millim. 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 labours of Nobert, Rutherfurd, 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 Prof. Rowland 

 that the direct measures of wave-lengths in the solar-heat 

 spectrum I have already made public f have been chiefly 

 executed. 



In Plate IV. fig. 1 we have a necessarily condensed represen- 

 tation 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 yu, (eighteen one-hundred-thousandths of a milli- 

 metre), which represents the shortest measured in the electric 

 spark from aluminum. Next, near' 0*29 fi (b) we have, 

 according to M. Cornu, the shortest solar ray which penetrates 

 our atmosphere ; near 0*35 \x (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 fi (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 



* 1-0 1*= 0-001 millim. 



f Comptes Mendus, Sept. 11, 1882 ; National Academy of Science, 1883; 

 Amer. Journ. of Science, March 1884; Phil. Mag. March 1884. 



