﻿S. P. Langley — Unrecognized Wave-lengths. 105 



wave-lengths of these bands, even to those who discovered them. 

 It is very likely that the (probably telluric) absorption band 

 in the solar spectrum placed on the Allegheny chart (Comptes 

 Eendus, Sept. 11, 1882) at 1^*38 had been recognized by more 

 than one of the above mentioned observers, yet so little was 

 known as to its actual position, even a few years since, that we 

 find the elder Draper, in reviewing these former observations in 

 1881, and speaking with the authority of one who was himself a 

 discoverer, expresses his doubt as to the possibility of any wave- 

 length so great as 1^*08 having really been observed, and M. 

 H. Becquerel (Annales de Chimie et de Physique, 1883, tome 

 xxx, p. 33) gives the wave-length of the longest band known to 



him as l^'SO. These remarks will not be superfluous as an in- 

 troduction to the following table, which presents a summary 

 view of the advances made beyond the above-named point in 

 the last five years. 



Extreme lengths of visible and invisible etherial radiations and 

 of sonorous waves. 



Quality of radiation 

 and means of 

 recognition. 



"Wave-lengths in 



units of 

 one millimeter. 



Description. 



Invisible ultra-violet 

 radiations. 



(Photography.) 



Visible radiations. 

 (Eye.) 



0-000185 



0-000295 



0-00036 

 0-00081 



Extreme rays of aluminum in the induc- 

 tion spark. Recorded by photog- 

 raphy. 



Extreme limit of solar spectrum at sea- 

 level on best days, according to 

 Cornu, recorded by photography. 



Limit of lavender light, visible to nor- 

 mal eyes. 



Extreme limit of deep red light, visible 

 to normal eyes. 



Beginni'g of infra-red 

 (Phosphorescence.) 



Photography, (Abney). 

 Pho sphor e scence , 

 (Becquerel). 

 (Bolometer.) 



o-ooio 



0-0015 

 0-0027 



Supposed extreme possible limit of 

 infra-red wave-lengths in 1881, ac- 

 cording to J. W. Draper. 



Wave-lengths assigned by H. Becquerel 

 to lowest absorption band known to 

 him in 1883. 



Sensible limit of solar infra-red rays 

 which penetrate our earth's atmos- 

 phere. Determined by the grating 

 and bolometer, Allegheny, 1882. 



