234. Dr. Miiller on the Thermal Effects 



green into blue (Pogg. Ann. vol. xxxv. p. 307). But neither in 

 this nor in a subsequent paper are the numerical data furnished 

 by the author. We find stated neither the magnitude of the 

 deflections of his thermo-multiplier, nor the width of the spectra 

 upon which he experimented. In short, in Melloni's memoir 

 there are no fixed data whatever from which the curve of thermal 

 intensity in the solar spectrum might be constructed. 



At first Melloni regarded the rays of heat and light as essen- 

 tially difi'erent; subsequently he pronounced his opinion distinctly 

 as to their being completely identical. 



The latter opinion, that rays of light and heat of the same 

 refrangibility are absolutely identical, is also advocated by 

 Masson and Jamin {Comptes Rendus, vol. xxxi. p. 14), who 

 state that they found that all rays of heat within the visible 

 spectrum are equally well transmitted by rock-salt, rock-crystal, 

 alum, glass, vvater, &c., and that the unequal diathermancy of 

 these substances wholly depends upon their possessing a differ- 

 ent absorbent power for those dark rays of heat which are less 

 broken than the red. There is probably no doubt as to the cor- 

 rectness of this result; still the above-mentioned physicists 

 should not have withheld the experiments which they consider 

 to have justified them in coming to this conclusion; for the 

 mere enunciation of the result obtained, without the adduction 

 of experimental evidence, can have little scientific value^ If 

 Masson and Jamin had published the observations which led 

 them to the above conclusions, we should doubtless have been 

 thereby provided with ample material to construct the curve of 

 thermal intensity, at least within the visible spectrum. 



To R. Franz belongs the merit of having been the fii'st to 

 publish data as to the amount of heat at different parts of the 

 spectrum, the measurements being performed by means of the 

 thermo-battery and the multiplier*. 



Although every physicist who has made the thermal effects 

 of the solar spectrum the subject of experimental investigation 

 must have observed that, in a spectrum which is pure enough to 

 show Fraunhofer's lines, the thermal effects are too small to 

 admit of anything like exact measurement, yet Franz was the 

 first to state this distinctly. 



The experiments of Franz were performed with a flint-glass 

 prism. The solar rays reflected from a metallic mirror entered 

 the dark room through a slit of 4 millims. breadth. 32 millims. 

 behind the first slit a second, 2 millims. in breadth, was placed, 

 the prism being adjusted close behind the second slit. 



At a distance of 17 centims. from the prism, the breadth of 



* R. Franz, " Untersuchungen iiber die Diathermansie einiger gefarbtcn 

 Fliissigkeiten " (Pogg. Avn. vol. ci. p. 4G). 



