208 Mr. S. P. Langley's Experimental Determination of 



With the danger of extrapolations presented to us in such 

 examples as have been cited, we shall not attempt to gene- 

 ralize the results of our observations further than to remark 

 that for the prism in question we find that the deviation tends 

 within the limits of observation to become proportional to the 

 wave-lengths as the deviation diminishes, and that, as far 

 as we can see at present, there is scarcely any limit to the 

 wave-length our prism can transmit except that fixed by its 

 absorptive effect. 



The approximate limit of the solar spectrum of the Hilger 

 prism is at n — 1*5435, which, according to Briot's formula, 

 corresponds nearly to 3^4, but which, according to our bolo- 

 metric observations, corresponds to an actual wave-length of 

 2^'S. For this same point, as will be seen by Table IV., the 

 values by Cauchy's formula are impossible, and those by Red- 

 tenbacher's formula are imaginary. 



These last values rest, it will be remembered, on extra- 

 polations founded on measures in the visible spectrum. 



Wave-lengths of Cold Lines in Infra-red Prismatic Spectrum. 



The following values (in Table V.) from Mouton, Abney, 

 and Draper are the ones I know previous to my own mea- 



Table V. — Observed Values of Cold Bands in Infra-red by 

 different investigators. 



M. Mouton*. 



W. deW. Abney t. 



J. W. DraperJ. 



S. P. Langley§. 





0-824 



0-815 0-835 



0-815+0-003 



0-850 



0-854 





0-85 +0003 







0-893 0-930 



0-89 +0004 





0-905 





0-91 +0-004 





0-941 



0-935 0-980 



0-94 +0-004 



0-985 



0-975 0-983 





1-13 +0007 



1-230 



1-240 





1-27 +0007 



1-480 



possibly Abney's " ip " 





fl-36 +0-008 

 1 1*37 +0-008 

 1-54 +0009 x 

 1-58 +0-009^ 

 1-81 +0-0101 

 1-87 +0-010/^ 

 1-98 +0010^ 

 2-03 +0010cu 2 



* M. Mouton, Comptts Rendus, tome lxxxix. p. 298 ; tome lxxxviii. 

 p. 1190. 



t W. de W. Abney, Phil. Trans. 1880, p. 653. 



% J. W. Draper, Proc. Amer. Acad. 1881, p. 223. 



§ S. P. Langley, Comptes Rendus, Sept. 11, 1882 ; Am. Journ. of 

 Science, March 1883 ; Phil. Mag. March 1883, &c 



