242 



Dr. Miiller on the Thermal Effects 



In the following Table the mean values are given, for the 

 values estimated in the above manner, of the thermal eflfects in 

 the different portions of the spectrum : — 



o 



Boundary of indigo and violet ... 2 



Middle of the blue 4 



Middle of the yellow 7 



Middle of the red 10 



1'" beyond the red boundary • . . .12 



2"' „ „ .... 11 



4'" „ „ .... 7 



6'" „ „ .... 2 



According to these observations, therefore, the maximum tem- 

 perature of the spectrum from crown-glass lies beyond the 

 boundaiy of the red, while Franz found this maximum for his 

 flint-glass prism to lie in the red itself. This difference, how- 

 ever, may be readily explained. The path which the rays had to 

 traverse in the interior of Franz's prism was about 18 milhms., 

 while I had so adjusted the slit in the tinfoil coating that the 

 path traversed by the rays in the glass was only 10 millims. 

 Inasmuch now as the glass acts as an absorbent for the thermal 

 i-ays, the thermal maximum must clearly undergo a displacement 

 towards the red when the thickness of the glass to be passed 

 through increases. 



The curve of 

 thermal intensity, 

 Rfl6S(fig.l), for 

 the spectrum of a 

 glass prism is con- 

 structed accord- 

 ing to the above 

 numerical data. 

 We see in this 

 figure that the 

 thermal elongation of the spectrum beyond the red occupies a 

 space almost as long as the whole of the visible spectrum. (The 

 latter is marked by a bracket in the figure.) In the visible por- 

 tion of the spectrum, the positions of the more important Fraun- 

 hofer^s lines are marked for perspicuity. 



According to the observations of Franz, the dark portion of 

 the thermal spectrum is nearly the same length as I have obsei-ved 

 it to be ; indeed, our observations in general either agree per- 

 fectly with one another, or differ only to an immaterial extent. 

 I considered it, therefore, unnecessary to institute a second 

 series of experiments with the glass prism, and proceeded to 

 employ the one of rock-salt. 



