Third Series. — Refrangihility of Heat. 185 



intermediate between total and partial reflection the intensities 

 ol' the reflected heat, and by constructing a curve having 

 measures of the diagonal of the lozenge (a function of the 

 angle of incidence) for abscissae, and intensities for ordinates, 

 I endeavoured to discover graphically for what vahie of the 

 former the measure of the latter increased most rapidly, in 

 other words, where the tangent made the greatest angle with 

 the axis, or where was the point of contrary flexure of the 

 curve. 



Plate IV. fig. 2, may represent such a curve. I have 

 found that when the diagonal of the lozenge was 14*5 inches, 

 the reflection was in all cases nearly total, or the galvanome- 

 ter was little affected by any increase of the angle of incidence. 

 This effect, measured by the vertical line A B, was denoted 

 by 100. When the diagonal was increased to 15*0, the effect 

 was reduced, we shall suppose, to 90, expounded by the line 

 C D, at 15'5 by E F, and so forth. An interpolating curve 

 drawn through the points so fixed, would have its greatest 

 inclination to the axis A X, when, for a given variation of 

 the diagonal, the decrement of the intensity was a maximum, 

 in other words, at the determining angle for the predominating 

 part of the heat used. Such a point of contrary flexure would 

 therefore determine the mean index of refraction of the given 

 kind of heat by the aid of the formula above investigated, 

 whilst the form of the curve would lead to some conjecture at 

 least, respecting the distribution of heat of the more or less 

 refrangible kinds in the given ray. Heat of low refrangihility 

 being the last to be totally reflected, would cause the curve to 

 droop fastest near the extremity B, the more refrangible rays 

 would be cut off' at the other end I of the curve. 



I lost no time in verifying the general truth of the principle, 

 and also of the received doctrines respecting heat, by ex- 

 amining the quality of the heat which reached the pile at dif- 

 ferent stages of total reflection. If, as M. Melloni first ren- 

 dered probable, heat of low temperature is least refrangible, 

 and vice versa', and further, if it be admitted that such heat 

 passes most difficultly through such substances as glass, it 

 follows, that after total reflection has proceeded a certain 

 way, so that the more refrangible, and therefore more trans- 

 missible, rays have suffered total reflection, whilst the remain- 

 ing rays constituting the primitive beam continue to be re- 

 fracted, the heat thus reflected will be more copiously trans- 

 mitted by glass, than when it came direct from the source. 

 This conjecture was precisely verified. 



Subsequent experiment still more fully confirmed this result, 

 and by showing that during the whole progress from partial 



