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



Quality of radiation 

 and means of 

 recognition. 



Wave-lengths in 



units of 

 one millimeter. 



Description. 



Invisible infra-red radiations from terrestrial 

 sources. (Bolometer). 



00053 



0-0075 



0-011 



0-030 



Limit of absolute measurement of wave 

 lengths corresponding to a given 

 index of refraction in the case of a 

 rock-salt prism. Determined by the 

 Rowland grating and bolometer, 

 Allegheny, 1886. 



Approximate position of the maximum 

 ordinate in the heat spectrum from a 

 lamp-blacked surface at the tempera- 

 ture of boiling water (100.°). Alle- 

 gheny, 1886. 



Approximate position of the maximum 

 ordinate in the '• heat" spectrum from 

 a lamp-blacked surface at the tem- 

 perature of melting ice (0°). Alle- 

 gheny, 1886. 



Approximate estimate of the minimum 

 value assignable to the longest wave 

 recognizable by the bolometer in any 

 heat from a rock-salt prism. 



Sonorous 



Vibrations. 



(Ear.) 



14-0 



Length of shortest sound wave corres- 

 ponding to highest musical note per- 

 ceptible by human ear. (Approx- 

 imately 48,000 s. v. per sec.) Savart. 



Broadly speaking, we have learned through the present 

 measures with certainty of wave-lengths greater than 0*005 mm , 

 and have grounds for estimating that we have recognized radia- 

 tions whose wave-length exceeds 0'03 mm , so that while we have 

 directly measured to nearly 8 times the wave-length known to 

 Newton, we have probable indication of wave-lengths far 

 greater, and the gulf between the shortest vibration of sound, 

 and the longest known vibration of the ether, is now in some 

 measure bridged over. 



In closing this memoir, I must add that the very considerable 

 special expenses, which have been needed to carry on such a 

 research, have been met by the generosity of a citizen of Pitts- 

 burgh, who in this case, as in others, has been content to pro- 

 mote a useful end, and declines publicity for his name. 



I cannot too gratefully acknowledge my constant obligation to 

 the aid of Mr. F. W. Yery and Mr. J. A. Keeler, of this Obser- 

 vatory, who have labored with me throughout this long work. 

 In the prolonged numerical and other computations rendered 

 necessary, I have been aided by Professor Hodgkins, of Wash- 

 ngton, and by Mr. James Page of this Observatory. 



Allegheny Observatory, May 31, 1886. 



