158 



MEMOIRS OP THE NATIONAL ACADEMY OF SCIENCES. 



cording to Briot's formula, corresponds nearly to 3//.4, bnt which accordiug to our boloineli ic 

 observations corresponds to an actual wavelength of 2//.8. For this same point, as will be seen 

 by Table 4, the values by Cauchy's formula are impossible, and those by Eedtenbacher's 

 equally' so. 



We may add that Briot's formula gives a point of inflection near \=2>^.(). In other words, 

 the curve which up to near the limits of our chart (Plate XI) has beeu convex to the axis of x, 

 there becomes concave. These values for Briot's formula rest, it will be remembered, on extra- 

 polations founded on measures in the visible spectrum. 



WAVE-LENGTUS OF COLD LINES IN INFRA-RED PRISMATIC SPECTRUM. 



The following values (in Table 5) from Mouton, Abney, aud Draper are the only ones I 

 know previous to my own measures where the wave lengths of any cold lines are given with 

 approximate accuracy. Of these it is just to distinguish those by Abney as possessing a degree 

 of exactness before unknown. There ai'e some doubts about the band l''.3G to 1''..37 having really 

 been observed before, but I have included this among those whose existence was known or 

 suspected before my measures. 



The values here given were obtained by me in 1882, aud first published in the Comptes 

 liendus of the Institute of France, for September 11, 1882, iu the form of charts, which were 

 drawn from them. (The original charts have been given here already on a reduced scale in Plate 

 XIX.) These charts were so much reduced by the first engraver that tliough these values are still 

 determinable from them, it may be convenient to repeat them here in their original tabular form, 

 with the addition of the probable errors. 



Table V. — Observed values of cold bands in infrared by different investigators. 



*M. Mouton, Comptes Re.ndus, tome Ixxxix, p. 298; tome Ixxxviii, p. 1190. 

 t W. de W. Abney, Phil. Trans., 1880, p. 6.53. 

 t J. W. Drapeh, Proc. Am. Acad., 1881, p. 223. 



§S. P. Langley, Comptes Rendus, Sept. 11, 1882; Am. Journal of Science, March, 

 1883, &c. 



LINES KNOWN TO PREVIOUS INVESTIGATORS. 



(0.815.) Near the utmost limit of visibility. Appears to coincide with Captain Abney's Z, 

 and Draper's a. 



(0.85.) Apparently agrees with Abney's 8540. 



(0.89.) An inconspicuous line. Abney" has a heavy line near here. Possibly corresponds to 

 Draper's fi. 



