On the Diathermancy of the Media of the Eye. 177 



degrees; but when both sides of the combination were blackened, 

 the action on the thermo-pile was zero*. 



In 1850, A. Cima published, in the August Number of the 

 Journal 77 Nuovo Cimento, an investigation of the absorption of 

 the media of the eye for heat, and found that, of 100 rays inci- 

 dent from a Locatelli , s lamp, only nine penetrated the three media 

 combined by Briicke. Of the dark rays (the most important in 

 reference to the question proposed above) Cima has taken no 

 account. 



Tyndall resumed the investigation in the year 1859. The 

 results obtained agree in general with those published by Briicke. 

 An accurate description of TyndalFs experiments has not, how- 

 ever, been given. Lastly, Janssen published observations on 

 the absorption of heat by the media of the eye. He used a 

 moderator lamp as a source of heat. About 8 per cent, of the 

 incident rays were absorbed by the eye, the media of which 

 were in part investigated separately. In order to determine the 

 thermocrose of the separate parts of the eye, Jannsen compared 

 their diathermancy with the diathermancy of water, using differ- 

 ent sources of heat. In these experiments a perfect concordance 

 was observed between the absorptive force of a layer of water 

 (between glass plates), and of an equally thick layer of any of the 

 media of which the eye is composed f. But since Melloni has 

 shown that obscure heat can penetrate water J, it would follow 

 from the last observations that obscure thermal rays can reach 

 the retina. But the heat which is present in the luminous rays 

 could not, according to Masson and JamhrVlaw§ (that perfectly 

 transparent bodies transmit all luminous zones of heat in the 

 same manner), be affected by the absorption which takes place in 

 the interior of the eye, provided that the media of the eye are 

 quite clear. Hence the greater part of the thermal rays absorbed 

 by the eye are obscure. 



I have obtained results similar to those of Janssen, but by the 

 application of the solar heat. 



A pencil of solar rays was reflected into a dark room, and then 

 decomposed by a prism in order that the zones of different wave- 

 length might be individually tested as to their transmission 

 through the media of the eye. The arrangement of the separate 

 pieces of apparatus was analogous to that described in the 

 Annalen, vol. ci. p. 47. The well-known property of rock salt, 

 of possessing the greatest diathermancy for all thermal colours, 

 recommends the use of a rock-salt prism in decomposing the 



* Poggendorff's Annalen, vol. lxix. p. 551. 

 f Ann. de Chim. et de Phys. vol. lx. p. 71. 

 | Poggendorff's Annalen, vol. xxiv. p. 645. 

 § Comptes Rendus, vol. xxxi. p. 14. 



