180 M. R. Franz on the Diathermancy of 



relation of the quantity of heat in the luminous zones after it 

 had traversed the medium, was very little different from that 

 which a layer of water about 60 millims. thickness had previously 

 given ; for the obscure zones, the mean of three series of obser- 

 vations gave, 



Red zone 100 



First dark zone .... 7*1 

 Second dark zone . . . 2*6 



The vitreous humour, lastly, for about the same thickness as 

 the aqueous humour, transmitted the heat of the different zones 

 in about the same proportion as the latter. 



It appeared probable, therefore, from these experiments, that 

 the non-luminous thermal rays could penetrate the media of the 

 eye, although to a very small extent. But as, from the defective 

 sharpness of the limits of the zones when rock salt was used for 

 the prism and for the sides of the trough, the above experiments 

 had not the certainty of an unfailing result, I decided to replace 

 rock salt by glass. It is clear that, by using glass, a smaller 

 quantity only of the obscure rays could reach the media of the 

 eye; but if in this case a perceptible quantity traverses the 

 media, it follows that the eye cannot be diathermanous for the 

 obscure rays*. 



The visible spectrum in the following experiments had an 

 extent of 18 millims. In the screen, which allowed only one 

 zone of the spectrum to fall each time on the thermo-pile and 

 retained the other zones, there was a slit of such breadth that 

 the thermo-pile itself was irradiated by a zone 3 millims. in 

 breadth. By a lateral motion of the screen and of the thermo- 

 pile, the different zones could be subjected to investigation. 



Placing the cornea in the rays from the prism, the following 

 values, for the quantity of heat transmitted, were obtained as the 

 mean from six series of experiments : — 



Violet 1-0 



Indigo 23 



Blue 3-7 



Green 7*3 



Yellow 15-2 



Red 100 



First obscure zone . . . 8*0 



Second obscure zone . . . 6-2 



Third obscure zone . . . 1-9 



* Volpicelli remarks in his Treatise on Radiant Heat (Atti de' nuovi 

 Lincei, vol. v. p. 145), that he found rock salt less diathermanous for solar 

 heat than flint glass. From comparative experiments which I have made, 

 and which are published in the above-mentioned ' Programme,' this is not 



