﻿S. P. Langley — Unrecognized Wave-lengths. 



95 



Header at circle, F. W". V. 



Header at galvanometer, S. P. L. 



Object = measurement of deviation of X = 4X^D 2 in the spec- 

 trum of the flint glass prism H. ISTo. 2 with sunlight. 



Galvanometer deflection with arms in line (from the combined 

 eflect of all the grating spectra falling on slit 2) a little over 300 

 divisions. 



Prismatic deviation. 



49° 00' 



48° 58' 



48° 56' 



48° 54' 



48° 52' 



48° 50' 



Galvanometer deflections. 

 First series 



n 



2 



10 

 13 



12 



18 

 15 



19 

 13 



12 

 15 



Mean deflections 



65 



11-5 



11-5 



16-5 



16 



13.5 



Galvanometer deflections. 

 Second series _ 



13 



14 



21 

 22 



26 

 24 



32 

 31 



25 

 30 



20 





13 



Mean deflections 



13-5 



21'5 



25 



3T5 



27-5 



16-5 



Concluded maximum at 48° 54'. 



Making the galvanometer deflections ordinates, and the 

 prismatic deviations abscissas, a smooth curve through the 

 points of observation gives, in the first case, a maximum of 

 48° 54/. The image of the slit has a certain size, and so has 

 the bolometer strip. The latter feels the heat before the center 

 of strip and image coincides, and it is this point of the coinci- 

 dence of centers which gives the maximum as denoted by the 

 above figures. 



We now make a second series, and though the two series 

 follow each other at a brief interval on a day described as 

 "clear," the values of the deflections in the second series on 

 the same points indicate nearly twice the heat in the first. 

 The change is chiefly due to the altered diathermancy of the ap- 

 parently clear sky in the brief interval. It is one of the difficul- 

 ties already signalized by the writer and by others,* and is to 

 be eliminated only by repeated observation. The second series, 

 however, gives the same deviation as the first, and hence we 

 conclude (as far as this day's observation goes) that this is the 

 index of refraction for the wave-length in question and for a 

 certain flint prism. Such observations must generally be 

 repeated on many days before a reliable result is reached ; and 

 in the case of the glass prism, which grows rapidly athermanous 

 just beyond the limit of the solar spectrum, they are limited, 

 by the nature of the substance, to little more than the wave- 

 length in question. 



* See Crova, Comptes Rendus, vol. ci, p. 418, Aug. 10th, 1885. 



