172 Prof. S. P. Langley on unrecognized Wave-lengths. 



Extreme Lengths of Visible and Invisible Etherial Radiations 

 and of Sonorous Waves. 



Quality of radiations 

 and means of recog- 

 nition. 



Wave-lengths 

 in units of 

 one milli- 

 metre. 



Description. 



Invisible Ultra-violet 

 radiations (Photo- 

 graphy). 



Visible radiations 

 (%e). 



f 0-000185 



{ 0-000295 



f 00036 

 1 0-00081 



Extreme rays of aluminum in the in- 

 duction spark, according to Cornu. 

 Recorded by photography. 



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. 



Beginning of Infra-red 

 (Photography). 



(Phosphorescence). 



f 0-0010 



\ 



1 0-0015 



0-0027 



Supposed extreme possible limit of 

 infra-red wave-lengths in 1881. 

 (According to J. W. Draper.) 



Wave-lengths assigned by H. Becquerel 

 to lowest absorption-band known to 

 him in 1884. 



Sensible limit of solar infra-red rays 

 which penetrate our earth's atmo- 

 sphere. Determined by the grating 

 and bolometer, 1882. 



•a 

 •c 



i 



s 



o . 



00 3 



s © 

 .2 S 

 "SJ3 



e£ 03 





 1— 1 

 © 



'oa 



> 



1— I 



0-0053 



0-0075 



0-011 



0030 



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 tem- 

 perature of boiling water (100°) ; 

 Allegheny, 1886. 



Approximate position of the maximum 

 ordinate in the i£ 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 the 

 " heat " spectrum from a rock-salt 

 prism. 



Sonorous vibrations 

 (Ear). 



14 



Length of shortest sound-wave corre- 

 sponding to highest musical note 

 perceptible by human ear. (Approxi- 

 mately 48,000 s. v. per. sec, Savant.) 



