44 M. A. Wullner on the Absorption of Light, 



y : z : w. And we in fact have 



by*-cz*-dw* = bif-cz*-(y Vn + zScf 

 _ -bi/-2cz' 2 -2ijz </2bc 

 = -( y Vb + z\/2cY'> 

 and the right-hand side being a perfect square, the condition of 

 contact is satisfied. 



2. In like manner we have the system 

 z = Q, x VYa + y \/2b + w Vd=0, ax* + by 2 + 6xy \/a~b-dw*=0, 



which gives 



ax* + by 1 + 6xy Vab- dw 2 



= ax*-\-by* + §xy */ab — (x \/2a + y */2bf 

 = —ax' 2 — by* + 2xy */ab 

 = -(xx/a-y^by; 

 so that, the right-hand side being a perfect square, the condition 

 of contact is satisfied. 



Cambridge, November 28, 1863. 



I 



VII. On the Absorption of Light. By A. Wullner*. 



N a congratulatory paper, written on the occasion of Professor 

 Gerling's fiftieth-year-Doctor Jubilee in Marburg, I communi- 

 cated some experiments on the spectra of the incandescent vapours 

 of iodine, bromine, and hyponitric acid. I was led to these experi- 

 ments by the notice of Mr. Thomson, in the Annates de Chimie 

 et de Phys. 3 ser. vol. lxii. p. 191, in which he communicates an 

 earlier idea of Mr. Stokes, which includes a mechanical explana- 

 tion of the beautiful observations of M. Kirchhoff on the absorp- 

 tion of light in flames, and of the absorption of light in general. 

 Mr. Stokes's reasoning is as follows : — The emission of light has 

 its origin in a periodic motion of the molecules of the body, 

 which is communicated to the aether surrounding them; the 

 emission of a definite quality of light proves hence that the mo- 

 lecules of the flame are in a definite periodic motion. Incan- 

 descent soda vapour, which emits yellow light corresponding to 

 the dark line D, will hence have such a periodic vibratory motion, 

 its particles will have a duration of oscillation corresponding to 

 that of the aether for yellow light. 



For just the same reason sodium vapour surrounding a source 

 of light must have a tendency to retain in itself light of the 



* Translated from Poggendorff's Annalen, Oct. 1863. 



