182 M. R. Franz on the Diathwmancy of 



The last series of experiments referred to the vitreous humour. 

 A trough was used like that for the aqueous humour. As a 

 mean of four series of observations, the following values were 

 obtained for the ratios of the quantity of heat transmitted : — 



Green zone 4*2 



Yellow zone 7*6 



Red zone 10*0 



First obscure zone . . . 9*2 

 Second obscure zone . . . 6*7 

 Third obscure zone . . . 2*9 

 Fourth obscure zone . . 0*5 



From these results it appears that the absorptive powers of the 

 different media of the eye are very similar to that of water; 

 only the cornea and the crystalline appear to absorb more red 

 rays than water does. Hence if we are to hold to the theory 

 of the identity of light and heat, we must look for the reason of 

 the invisibility of those thermal rays of less refrangibility than 

 the red rays in the nature of the retina — revert, in fact, to a 

 supposition like that of Melloni : that the optic nerve cannot 

 give back the slow oscillations of the obscure rays. 



I may here be permitted to call attention to a great difficulty, 

 which diminishes the accuracy of results obtained with the ob- 

 scure rays of the sun. Even if atmospheric air exercises but a 

 small absorption on radiant heat, and air saturated with aqueous 

 vapour has the same diathermaneity as dry air*, yet there are in 

 our atmosphere so many fog-vesicles and finely-divided solid sub- 

 stances, that a great quantity of the thermal rays of the sun is 

 lost, either by reflexion or by absorption. Hence, with an appa- 

 rently clear sky, the distribution of heat in the various spectral 

 zones is very different. I may here adduce two series of obser- 

 vations, the most favourable and unfavourable of several, which 

 were to determine the heat of the zones of the spectra, using a 

 rock-salt prism, and both obtained with an apparently clear sky. 



August 12. August 21. 



Red zone .... 10*00 lO'OO 



First dark zone . . 8-94 13-00 



Second dark zone . 1*11 8*95 



Third dark zone . . 0*26 4*86 



Fourth dark zone . ... 1*36 



Fifth dark zone 0'73 



Sixth dark zone . ... 0*07 



* Magnus (Pogg. Ann. vol. cxii. p. 540). According to Tyndall's inves- 

 tigations (Pogg. Ann. vol. ex. p. 13; Phil. Mag. vol. xxii. p. 276), air satu- 

 rated with aqueous vapour absorbs a portion of heat. Tyndall (Pogg. Ann. 

 vol. cxiv. p. 632; Phil. Mag. vol. xxii. p. 377). Magnus (Pogg. Ann. vol. 

 cxiv. p. 635 ; Phil. Mag. vol. xxiii. p. 250). 



