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



source, observed in any prism; and thus to gain that knowl- 

 edge of the intimate constitution of radiant bodies which an 

 acquaintance with the vibratory period of their molecules 

 can usually alone afford us. It is this considerable end, — the 

 opening up to research of the whole unexplored region of 

 infra-red energy, not only from celestial but from terrestrial 

 sources — which will, we trust, justify the labor devoted to the 

 following determinations. It may be hoped that wider inter- 

 est will attach to our task of demonstrating the character of a 

 certain curve, when it is seen that a knowledge of its true form 

 has ceased to be a matter of abstract speculation only, but will, 

 in connection with what has already appeared, now introduce 

 to us such large regions of research as we have just indicated. 

 Over and above all this, however, we shall find our results also 

 affecting opinion on the theoretical considerations regarding 

 the relation of wave-length and dispersion just alluded to. 

 In previous communications I have given a representation 



of the solar heat spectrum terminating near 2 /z, 7 or 2^'S and I 

 have stated that while there were feeble indications of solar 

 heat below this point, yet that the solar radiation beyond 

 seemed sensibly cut off, as though below this were a nearly un- 

 limited cold band. I do not mean then, in saying that solar 

 heat sensibly ceases below this point, to say that absolutely none 

 can exist, but that none at any rate does exist sensible to the 

 delicate apparatus with which these first determinations were 

 made, and that none in any case exists of an order comparable 

 with the smaller portions of that already described. 



The reader will gather a more clear conception of the difficulty 

 of decision and of the almost infinitesimal amount of this solar 



heat below 3^, if it exist, by looking at Plate I in connection 

 with the statement, that if there be any solar heat at 4^, the 

 highest ordinate representing it on the same scale as that 

 shown on the left of the Plate, would at any rate not occupy 

 the thickness of the horizontal line which represents the 

 axis of abscissas. However, since we are rather inclined to 

 admit from our final experiments with our latest and most 

 sensitive apparatus, that heat of some kind reappears here 



near 4/*, whether from the atmosphere of the sun, or elsewhere, 

 insensible to the thermopile, and in any case, if it be real, 

 almost infinitesimal in degree, or of the same order of intensity 

 with that in the lunar spectrum; our statement that no sensible 

 solar heat exists here must be taken under this qualification. 



New Apparatus. 

 The apparatus for the determination of wave-lengths in con- 

 nection with the flint glass prism has been already described.* 



* See this Journal, March, 1884. 



