﻿S. P. Langley — Unrecognized Wave-lengths. 103 



within the very extended limits in which we have followed it, 

 the contrary happens, and the curve presents an increasingly 

 constant angle with that axis. 



These results, in some material points, are in contradiction 

 to what has usually hitherto been believed.* 



Let me repeat that one consequence of the fact that the curve 

 is approaching a straight line is that, unless there is some 

 immediate change in its character, such as we have no right to 

 expect, extrapolation considerably beyond the point to which 

 we have measured will be comparatively easy and safe. I am 

 aware of the danger attending all extrapolations, but I must in- 

 sist upon the fact that the old ones, which we have falsified by 

 experiment, rested on extremely limited regions of the curve, 

 that, namely, for the visible region of the spectrum, in which 

 the relation between n and X is also wholly different; while 

 those, on which we now briefly enter, depend upon far greater 

 material for induction (about eight times that included in the 

 visible spectrum), which we can also use under much more 

 favorable conditions. 



Since the curve still presents a slight convexity to the axis 

 of abscissas, unless its character changes in a way which we 

 have no ground to expect, a tangent at any point will meet that 

 axis sooner than the curve itself will. Accordingly if we now 

 ask what wave-length corresponds to any point in the hitherto 

 unexplored region, for instance, the maximum in the spectrum 

 of boiling water whose indexf for the rock-salt prism is 

 1*5145 J or that of melting ice whose index is 1*5048, we can 

 answer as follows : First, this unknown wave-length is at any 



rate greater than 5^-3 since to this point we have investigated 

 by direct measurement ; second, since the tangent to our curve 

 even at the point 5^ meets the line corresponding to the index 

 of the maximum heat in boiling water at over 7^ and a line 

 corresponding to the maximum ordinate in the spectrum of 

 melting ice at over 10*", and since the curve without some 

 change in its essential character cannot meet these save at 

 still greater wave-lengths, it follows that the wave-length of the 

 maximum of the spectrum from boiling water is probably at 

 least *0075 mm , and that of the maximum in the spectrum from 

 melting ice is over 0*01 mm . In an article in the Comptes 



* I am very desirous that they should be verified by physicists, and I have there- 

 fore given particulars in some detail of my methods and apparatus here. I have 

 requested the skillful artists (Mr. W~m. Grunow, mechanician to the IT. S. Military 

 Academy, TVest Point, N. T., Mr. J. A. Brashear, Allegheny, Penn.) who have so 

 successfully constructed this apparatus, to place at the command of physicists all 

 or any details of it. 



f Determined as described in this Journal for January, 1886, by means of the 

 apparatus shown in Plate II. \ See Plate III, this Journal for Jan., 1886. 



