34;2 Dr. Seebeck on the unequal Evolution 



By the results of these last experiments those of the former 

 were not only confirmed, but obtained additional weight ; for 

 it appears from them, that the differences have been found 

 rather too small than too great ; since the elevation of the spec- 

 trum above the normal station, whether effected by the turn- 

 ing of the prism or the change of position of the sun, could 

 not but have an injurious effect on the results obtained. 



Thus the differences of temperature in the different pris- 

 matic colours will be most distinct with the normal position of 

 the prisms, in clear days without exception. But it is different 

 when the atmosphere is impregnated with vapours, especially 

 when they begin to collect before the sun. For then I found 

 repeatedly, even in this favourable position of the apparatus, 

 but very small differences in the heat of colours closely bor- 

 dering on one another, especially also between those of the 

 red and yellow half of the spectrum. 



The light of the sun then assumes a yellowish tinge. It 

 was to this circumstance that I thought the deviating effect 

 should be chiefly attributed. This occasioned the following 

 experiments : 



Exp. 52. — A prism of white Bohemian glass, which in clear 

 days always gave the greatest heat in the red (viz. No. 4 of the 

 above), in the normal position, at a distance of six feet from 

 the thermoscope. Its station in the place of experiment being 

 5" 1 1'". 



a. An oranffe- coloured glass fixed to the inner surface of 

 tlie prism. 



In full red 8" 81'" 



Close under the red 810 



b. The same glass fixed against the outside face of the 

 prism ; an hour later. 



In the red 8"]]'" 



Immediately below the red 9 



Exp. 53. (On another day.) The same prism, apparatus, 

 and distance. Station of the thermoscope in the room where 

 the experiment was made, 5". The orange-coloured glass out- 

 side the prism. 



rt. In the red 6" 7'" 



Close under the red 7 2 



b. In the red 6 7 



Close under the red 7 2 



The heat under the red was thus always greater, but the 



difference very unequal. 



c. Without a coloured glass. 



In the red 10" 4.'" 



Close under the red 10 4 



These 



