178 M. R. Franz on the Diathermancy of 



sun's rays, when it is a question of working with the largest 

 possible number of those rays. In fact, direct comparison of the 

 spectra of a flint-glass and of a rock-salt prism shows that the 

 heat in the bright zones of the spectra produced by both prisms 

 has equal intensity, but in the dark zones an increase in favour 

 of the rock-salt prism, as I have elsewhere shown*. Neverthe- 

 less the absorption of obscure heat by the vesicles of fog, with 

 which, in our latitudes, the atmosphere is perpetually charged, 

 is so great that the differences in the quantity of heat of the 

 individual obscure zones is not so pronounced as might have been 

 expected from Melloni's investigations, which were made under 

 more favourable atmospheric conditions. With a perfectly trans- 

 parent rock-salt prism, and with an apparently perfectly clear 

 sky, I have never been able to prove an action beyond the sixth 

 dark zone — that is, never at a greater distance from the red limit 

 of the spectrum than the extent of the visible spectrum. But the 

 maximum of the thermal action, using a rock-salt prism, is found 

 in the first dark zone, while, using a flint-glass prism, the maxi- 

 mum is in the red. 



At the beginning of these investigations on the absorption of 

 the separate media of the eye, only the results obtained by Briicke 

 were known ; and these led me to avoid all glass envelopes, in 

 order to be able to prove the presence of the quantity of obscure 

 heat, in any case small, which penetrates the media of the eye. 

 With this view, the moist parts of the inner eye were poured 

 between two transparent rock-salt plates, and the separate zones 

 of the thermal spectrum produced by a rock-salt prism were in- 

 vestigated. Only during a short series of experiments did the 

 spectrum remain on the screen of the thermo-pile sufficiently 

 clear to be used; for the liquid in the rock-salt trough soon dis- 

 solved part of the rock salt, by which irregular refractions were 

 produced. Hence the plates could only be used for one series of 

 experiments. But even the rock-salt prism, although freshly 

 polished before each series of experiments, soon exhibited a less 

 sharp refraction than was necessary for the experiment. 



The parts of the eye investigated were in all cases taken from 

 the eye of a recently-killed ox. By an equatorial section the 

 eye was separated from all muscle ; the vitreous humour could 

 then be obtained free of any turbidity from the pigment. 

 The upper part of the sclerotic, with the cornea, was then sepa- 

 rated from the pigment and stretched. It was, however, soon 

 found that this stretching did not form such a flat surface as to 

 produce on the other side a sharp picture of the spectrum. 

 Hence afterwards, in using the cornea, a small metallic cylinder 

 was used, which, closed at one end by a transparent plate, was 



* Programm of the Berlin Gymnasium zum rjrauen Kloster, Easter 1858. 



