[ 176 ] 



XXV. On the Diathermancy of the Media of the Eye. 



By R. Franz *. 



THE spectrum which is produced by a clear prism exhibits 

 to the eye the greatest intensity of illumination, and hence 

 the maximum of luminous action, in the yellow. Only in a few 

 cases do our thermometric apparatus show the maximum effect 

 of the spectrum in the yellow, but often in the red, and often 

 also outside the red : they even prove that the sun's action in 

 favourable circumstances only ceases at a distance from the red 

 which considerably exceeds the distance of the extreme chemi- 

 cally active rays. How is this phenomenon to be explained on 

 the assumption which so many experiments, and the concurrence 

 of eminent observers in this department has made probable, the 

 theory, namely, that light and heat are identical? Melloni en- 

 deavoured to explain the phenomenon of the great intensity of 

 illumination in the yellow by assuming that the retina of the eye 

 has a yellowish colour, which in age becomes pale, and is then 

 replaced by a yellowish coloration of the crystalline lensf. The 

 first part of this assumption would express the conjecture that 

 the retina is more susceptible to the vibrations of the yellow 

 light — in a certain sense, harmonizes more with them than with 

 vibrations of greater or less wave-length. 



To this hypothesis of Melloni, Briicke opposed a series of accu- 

 rate observations. He imagined that the media of the eye do 

 not transmit rays of great wave-length, and therefore that an 

 excitation of the retina by them cannot take place. In order to 

 establish this supposition, he used an Argand burner as a lumi- 

 nous source of heat, while the same lamp covered by a black sheet- 

 iron cylinder furnished the obscure source of heat. A deflection 

 of the needle of the galvanometer, amounting to from 40° to 50°, 

 was observed while the bright source irradiated the thermo-pile 

 directly ; but when the cornea was interposed, the deflection was 

 only 8 to 9 degrees; and the deflection was reduced to 1J degree 

 when the crystalline lens was interposed. When both were 

 simultaneously interposed, no thermal action was produced. 

 When the dark cylinder was placed round the lamp, the galva- 

 nometer exhibited no deflection, either when the crystalline lens 

 or the cornea was placed in the path of the thermal rays J. 



Briicke subsequently repeated these experiments in another 

 manner, by placing a dioptric combination of vitreous humour, 

 lens, and cornea in the path of a pencil of solar rays. The de- 

 flection of the galvanometer amounted in this case to from 26 to 30 



* Translated from Poggendorffs Annalen, February 1862. 

 t Poggendorff's Annalen, vol. lvi. p. 574 et seq. 

 % Ibid. vol. lxv. p. 598. 



