264 M. R. Riihlmann on the Alteration produced by Heat 



Eraunhofer's results — much more so than mine. This fact, taken 

 in conjunction with the great discrepancy between different ob- 

 servations made under like circumstances, throws an unfavour- 

 able light upon their accuracy. On this account it was not pos- 

 sible for me to regard the problem as solved — the less so because 

 Gladstone and Dale had not even taken the trouble to deduce a 

 lawin the form of an interpolation formula from their experiments. 

 It is not possible to determine the source of the above pointed 

 out inexactness, because the numbers upon which the results are 

 based are nowhere given, and we are thus debarred from the 

 detection of any possible considerable error arising from inad- 

 vertence or miscalculation. 



The next investigation which must, at all events for the sake 

 of completeness, be mentioned is by Schmidt*, who had occa- 

 sion to determine the line D for water and certain saline solu- 

 tions. In the first place, the graduation was insufficient on the 

 circle by which the angular measurements in these experiments 

 were made. In the second place, the hollow prism containing 

 the liquids was taken to pieces from time to time, its angle, 

 however, being determined only at starting, and there being no 

 guarantee of its remaining constant during the observations. 

 The numbers obtained in this manner certainly deviate enor- 

 mously from all others ; for instance, for D, 



ForO°-9C 1-3355 (Sch.) instead of 1-3337 (R.). 



Although Dr. Schmidt was obliged himself to admit the un- 

 trustworthiness of his results, he still decided to explain the 

 want of accord by the assumption that the sodium-line which he 

 used for D did not coincide with D, but lay so between D and 

 E that its distance from D was to its distance from E as 135 to 

 92. If we consider how thoroughly well established, not only 

 by Fraunhofer but by Kirchhoff, is the coincidence of the sodium- 

 line with the characteristic double line D, we may conversely 

 conclude how little weight is to be attached to Schmidt's deter- 

 mination of the refractive indices. 



Much greater importance is to be attached to the observations 

 of Landolt on the alteration of the refractive power with the 

 temperature, which are contained in his valuable experimental 

 research " On the Influence on the Velocity of the Propagation 

 of Light exercised by the Composition of Liquid Compounds 

 containing Carbon, Hydrogen, and Oxygen "f. His observa- 

 tions relate chiefly to their different temperatures and the throe 



* Pogg. Ann. vol. cvii. pp. 204 &c. se Auszug aus dem Programm des 

 Gymnasiums imd der damit verbundenen Realselmle zu Plauen fur das 

 Jahr 1859." 



t Pogg. Ann. vol. exxiii. p. 595, 



