On the Ultra-red Region of the Spectrum of the Sun. 235 



A similar result is obtained by causing the point of support 

 of the pendulum to vibrate in a vertical path. If we denote 

 this motion by 97 = /3sin2p£, the effect is as if gravity were 

 variable by the term 4p 2 /3 sin 2pt. Of the same nature are 

 the crispations observed by Faraday and others on the surface 

 of water which oscillates vertically. Faraday arrived experi- 

 mentally at the conclusion that there were two complete vibra- 

 tions of the support for each complete vibration of the liquid. 

 This view has been contested by Matthiessen*, who main- 

 tains that the vibrations are isoperiodic. By observations, 

 which I hope to find another opportunity of detailing, I have 

 convinced myself that in this matter Faraday was perfectly 

 correct. The vibrations of water standing upon a horizontal 

 glass plate, which was attached to the centre of a vibrating 

 iron bar, were at the rate of 15 per second when the vibra- 

 tions of the bar were at the rate of 30 per second. The only 

 difference of importance between this case and that of the 

 pendulum is that, whatever may be the rate of vibration of 

 the plate, there is always possible a free water-vibration of 

 nearly the same frequency, and that consequently no special 

 tuning is called for. 



XXXIY. On a Measurement of Wave-lengths in the Ultra-red 

 Region of the Spectrum of the Sun. By Eknst PuiNGSHEiMf. 



IN order to investigate the solar spectrum it is of special 

 importance to know the wave-length of the extreme rays 

 emitted by the sun, and thus to determine the extent of the 

 entire spectrum. In order to determine the wave-length of 

 the least-refrangible rays, Miiller|, and subsequently La- 

 mansky§, observed with the aid of a thermopile the index of 

 refraction of the extreme rays of a spectrum projected by a 

 crown-glass or rock-salt prism, and from that index calculated 

 the wave-length by means of an empirical formula, the cor- 

 rectness of which was controllable only within the limits of 

 the visible rays. The untrustworthiness of this method is 

 obvious; thus, from the same observation Miiller calculated, 

 by two different formulae, for the extreme wave-length the 

 values 0-00177 and 0*0048 millim. 



A trustworthy determination of the wave-length is possible 

 only with the aid of the interference of the rays ; and this 

 course was taken by Abney||, who succeeded in photographing 



* Pogg. Ann. cxli. 1870. 



t Translated from Wiedemann's Annalen, 1883, No. 1, pp. 32-45. 

 ; Pogg. Ann. cv. p. 352 (1858). § Ibid, cxlvi. 1872. 



|| Phil. Trans. 1880, pp. 653-667. 



