﻿100 jS. P. Langleij — Unrecognized Wa/oe-lengths. 



In the following brief 

 of all this labor. Our 

 terms of the wave-length, 

 known, and the latter the 

 mean probable error as 

 latter. 



table we have summarized the results 

 working method gave the index in 

 but since ordinarily the former is the 

 unknown quantity, we here give the 

 finally corrected as a function of the 





Wave-Lengths from 



Given Indices of Refraction 



Direct Observation, 



in Rock-Salt Prism. 



(a) by the eye. 





(b) by the bolometer. 



1-5442 



( AD 2 = 0^5890 ± 0-000 (a) 



1-5301 



2 X AD 2 = 1-1780 ±0-002 (b) 



1-5272 



3 X AD 2 = 1-7670 ± 0-005 " 



1-5254 



4 X AD 2 = 2-3560 zfc 0*009 " 



1-5243 



5 X AD 2 = 2-9451 ± 0*013 " 



1-5227 



6 X AD 2 = 3-5341^0-019 " 



1-5215 



7 X AD 2 = 4-1231 d= 0*029 " 



1-5201 



8 X AD 2 = 4*7121 ± 0*043 " 



1-5186 



9 X AD 2 = 5*3011 ± 0*065 " 



In Plate IY, we have graphically constructed the relations 

 between n and X for the rock-salt prism, as far as the above 

 wave-length of 5^ '3011 or 0-0053 mm . The ordinates are pro- 

 portional to the indices of refraction given on the axis of Y, 

 the abscissas to the wave-lengths on the axis of X. The two 

 vertical dotted lines carry the eye down to the corresponding 

 portion of the spectrum, which is visible. Between these lines, 

 lie the points of the visible spectrum observed on, and the dotted 

 curves show the results of extrapolation by various formulas. 



The actual points settled by observation are certain multiples 

 of the wave-length D 2 (0-0005890 mm ) and a small circle whose 

 diameter equals a unit in the third decimal place of the scale 

 of ordinates (indices) gives the position fixed by observation, 

 while the distance from the center at which the smooth curve 

 cuts the little circle furnishes a graphic presentation of its dif- 

 ference from observation. The labors of the past year, then, 

 have enabled us to absolutely and directly measure the index 

 of refraction of rays whose wave-lengths are greater than 

 Q.QQgnm or more exactly, which reach 53011 of Angstrom's 

 scale, and to do so, with an error which is probably in most 

 cases confined to the fourth decimal place of the index. As 

 we shall see more clearly by Plate IY, the relation between n 

 and X has changed from that apparently complex one we see in 

 the visible spectrum, so that n becomes almost a simple linear 

 function of A, and the results of extrapolation grow to a higher 

 order of trustworthiness than when made from points in the 

 visible spectrum alone. 



It appears to us that no formula of dispersion with which we 



