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XIX. On hitherto unrecognized Wave-lengths. 



By Prof. S. P. Langley, Allegheny, Pa* 



[Plates IV.-VI.] 



Y\7"E have already f presented a description of the method 



"by which we are able to fix the wave-lengths in the solar 



spectrum- by direct measurement as far as about 23,000 of 



o 



Angstrom's scale. At this point the heat in the solar normal 

 spectrum had become so feeble that it taxed the utmost limits 

 of our capacity in 1881 to measure it ; for it will be remem- 

 bered that we are able to study the prismatic spectrum of the 

 infra-red with comparative ease, because the prism condenses 

 the heat ; but the grating greatly diffuses and weakens it, so 

 that, were it due to this cause alone, we should find measure- 

 ments in this part of the grating spectrum enormously more 

 difficult than those in the prismatic. But, independently of 

 this, of the heat which belongs to any ray, our grating in 

 general employs not over the tenth part. These causes com- 

 bine to make the heat in certain portions, where we have been 

 compelled to measure, almost infinitesimal. 



We are led to take this labour ; not primarily to settle the 

 theoretical questions involved in determining the relations 

 between dispersion and wave-length (though these are most 

 interesting), but with the object of providing a way which 

 will hereafter enable any observer to determine the visible or 

 invisible wave-lengths of any heat, whether from a celestial 

 or terrestrial source, observed in any prism ; and thus to gain 

 that knowledge 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 attempt to open up more fruitful means of research 

 in the whole unexplored region of infra-red energy, not only 

 from celestial but from terrestrial sources — which will, we 

 hope, justify the labour devoted to the following determina- 

 tions. It may be hoped that wider interest 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 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. 



* Communicated by the Author, 

 t This Journal, March 1884, p. 194. 



