450 Dr. Seebeck on the unequal Evolution 



Of Landriani's and Senebier's experiments we know but 

 little. Senebier says that the heat in the red was always greater 

 than in the violet, but sometimes greater in the yellow than in 

 the red. He seems to have used a mercurial thermometer 

 •without the bulb being blackened, whence the differences proved 

 very small : for instance, between the violet and the red only 

 y^^th degree of Reaumur. He says his prism was English, but 

 whether of flint- or crown-glass, he does not state. 



From all these remarks on the investigations of these na- 

 tural philosophers, it will be seen that the contradictions in 

 their observations and assertions, apparently so important, are 

 in effect trifling; but that several results which, considered 

 singly, seemed to contradict each other, when compared with 

 otliers and arranged with similar ones serve to confirm new 

 facts, or such as have not yet been sufficiently noticed. 



I will, however, now proceed to prove by experiments (also 

 important in other respects) that the above positions derived 

 from my observations do not embrace all cases. 



I had a prism of common white glass, which I had ground 

 for a particular purpose on two sides. One of those surfaces 

 was yet however sufficiently bright that, when the rays of 

 the sun passed through it and the third side which had kept 

 its polish, a spectrum of tolerably brilliant colours was pro- 

 duced. With this instrument I made several experiments. 

 All gave the same result; the heat below the red being always 

 greater than in the red, by 3, 5, 8 and 10 lines, according as 

 the distance was greater or smaller, and the atmosphere more 

 or less free from vapours. I now had the dull surface of the 

 same prism polished, so that the refraction took place through 

 two polished sides ; and in all the experiments made with it, 

 the maximum of heat fell as decidedly within the red, as, 

 in the former state of the prism, it had invariably fallen with- 

 out it. 



The question now is : whether the limits of the prismatic 

 red were the same in both states of the prism? I had re- 

 peatedly noticed during the use of common prisms, that when 

 the sun was suddenly clouded, yet so thinly that the spectrum 

 was still discernible, the latter was always smaller than when 

 the sun shone with undiminished brilliancy. In order to con- 

 vince myself whether this was also the case with the ground 



Ruhland and Gehlen my experiments on crown-glass, flint-glass, and water, 

 and requested them to examine whether the flint-glass of Benedictbaj'ern 

 (of which the Roj'al Academy in Munich possesses some excellent prisms) 

 would produce the same result as those of English flint-glass, which I had 

 employed. But I have not learned the result. At that time M. Ruhland 

 had not made any experiments on the heat of the spectrum. 



prism. 



