METHOD OF EXAMINING DISPERSIVE POWERS. 35 



Count Rumford attempts to explain why there was lefs Remarks. 

 ice melted in fuch experiments as the 45ih than in thofe like 

 the 39th, and attributes the diminution of the effect to the de- 

 fcending currents, occafioned by the cold mixture furrounding 

 the warm one, which he thinks would obftrud the oppofite 

 ones afcending from the ice. But the effect in the 51ft, com- 

 pared with the 53d, being juft oppofite, hepaffes over without 

 explanation. — I have no doubt myfelf, but that the true caufe The differences 

 of the differences in both cafes, is to be found in the column n^n tempera- 

 expreffing the mean temperature of the water, and not in that ture of tho 

 expreffing its fituation, which I confider as having nothing to water ' 

 do in the bufinefs, but as it affects the general temperature. 

 The maximum effect with cold water will be when it is of the 

 temperature of about 48° or 50°, and the minimum above it 

 probably about 100° or 120° ; and in proportion as the mean 

 temperatures, in any experiment, deviate from thofe points, 

 the effects vary accordingly, let other circumftances be what 

 they may. 



Thus I have attempted to explain the rationale of thefe very Conclufion, 

 curious and interefting experiments, in a manner different to 

 what their ingenious author has done. And muft now leave 

 it to the reader to form his opinion. 



A Method of examining refraclive and difperfive Powers, by prif- 

 matic ReJleSiion. By William Hyde Wollaston, M. D. 

 F. R. S. * 



J.N examining the power with which various fubftances re- New method of 

 fraa and difperfe light, I have for fome time part employed a SSsJjSi 

 method unnoticed by writers on optical fubjects ; and, as it is difperfmg power* 

 not only convenient in common cafes of refraction, but alfo .. bodies * u Po R 

 capable of affording refults not attainable by other means, I 

 have been induced to draw up a ftiort account of the method 

 itfelf, and of the moft remarkable inftances of its application. 



This method was fuggefted by a confideration of Sir Ifaac 

 Newton's prifmatic eye-glafs, the principle of which depends 

 on the reflection of light at the inner furface of a denfe re- 

 fracting medium. 



Since the range of inclination within which total reflection by angle of total 



• ttutiiM r i™« reflection at the 



# Philof. Tranf. 1802. 



takes 



