380 Baron Wrede on the Velociti/ of Radiant Heat. 



then the pile was placed to the east of the telescopic axis, and 

 the intersecting point of the two threads was brought over the 

 western edge of the sun's disc. It follows from above, that 

 in this position the eastern edge of the disc was a tangent to 

 the line passing through the points of verticality of the pile. 



In this position was the apparatus allowed to remain unin- 

 terruptedly for five minutes by the aid of a screw attached to 

 the parallactic stand. The position of the galvanometer at- 

 tached to the pile was next observed, and noted for every 

 semi-oscillation*. The average of these thirty galvanometri- 

 cal determinations gave, therefore, a measure of the tempera- 

 ture at the eastern edge of the sun's disc. The box was then 

 turned round 180°, and the experiment continued in the same 

 manner, only that now the eastern edge fell on the crossing 

 threads, and the western edge touched the pile. 



This experiment was repeated several times, the pile being 

 after every time approximated one screw-length nearer to the 

 vertical thread by turning the micrometer-screw once com- 

 pletely round. Hence each pair of such observations could 

 independently of the others decide the question, whether heat 

 and light possess an equal or a diiferent velocity of propaga- 

 tion, and which of these is the greater. 



To obtain a quantitative measure of the difference of the 

 two velocities, it was necessary to compare all the observations 

 together. By expressing through interpolation the tempera- 

 tures of the eastern and western edges as a function of the 

 distance from the solar image measured in turns of the micro- 

 meter-screw, it could hence be concluded how much more or 

 less the screw ought to have been turned in order to obtain an 

 equal temperature in both cases. The half of this measure- 

 ment must therefore show how far the thermal is distant from 

 the luminous image: and this magnitude reduced to its equi- 

 valent angle gives the difference of the thermal and luminous 

 aberrations. 



Such an inquiry needs of course a great number of obser- 

 vations in order to arrive at a satisfactory result. The author 

 complained, that although the apparatus was mounted three 

 weeks before his departure from Stockholm, he was prevented 

 by unfavourable weather from making more than two sets of 

 observations, and of which only one took place under quite 

 favourable circumstances. In this set six pairs of observa- 

 tions were made with different positions of the screw, and by 

 all these observations the temperature of the eastern edge of 



• This galvanometer has been noticed in vol. xlii. p. 308 of Poggendorff", 

 note ; the deviations of this instrument are observed by means of a mirror 

 and a telescope. 



