190 Prof. Forbes's Researches on Heat. 



the commonly assigned dispersion of light, which for rock-salt, 

 is between the limits r54- and 1*57 nearly. This, however, 

 is for extreme rays of light, which can hardly be said of heat ; 

 the extremes of dispersion are certainly much wider apart. 

 (2.) The meafi refractive index of direct rays from different 

 sources varies surprisingly little. In fact the differences for 

 direct rays of heat from the Locatelli-lamp, incandescent 

 platinum, and from a crucible heated to 450°, seem almost 

 insensible, or within the limits of error of experiment. It is 

 to be recollected, however, that this is compatible with the 

 utmost variety in the composition of each. (3.) The effect of 

 interposed screens in modifying the transmitted heat is very 

 remarkable. These, so far as I have tried them, invariably 

 raise the index of refraction, (alum, glass, opake glass, and 

 opake mica for the Locatelli-lamp ; glass and opake mica for 

 incandescent platinum, and clear mica for dark heat). This 

 is the case even with those substances which suppress light 

 altogether, and which therefore cannot be considered to do 

 more than detach the heat of considerable refrangibility from 

 the light which usually accompanies it, not as stopping the 

 most refrangible rays and admitting the passage of those of 

 lower temperature. Probably no substance acts in this way, 

 though some (as black glass and mica, as the experiments of 

 Melloni indicate) may probably absorb the heat spectrum at 

 both extremities. It is probably to this source that we must 

 attribute the very small fraction of heat transmitted by the 

 black glass I used, being only that constituting the rays of 

 the higher degrees of refrangibility, all those of Tow and mean, 

 and also of the highest, degrees of refrangibility being pro- 

 bably absorbed. (4.) With respect to the homogeneity of 

 different kinds of heat, we can deduce nothing certain from 

 the forms of the curves. They confirm, however, a view 

 which I have long entertained, that heat from non-luminous 

 sources is more homogeneous than any other. I argued this 

 partly on the ground stated in p. 1 1 1 of this volume, and still 

 more from the uniformity of results which I have in all classes 

 of experiments obtained from dark heat, which often more 

 than made up for the narrower range of the thermal effect, and 

 which showed that the discrepancies observed in other cases 

 were due not so much to errors of observation, as to unavoid- 

 able changes in the character of the heat, (p. 100). This re- 

 sult is the more probable from the size of the source of heat 

 necessarily used in the crucible experiments (p. 188), which 

 tends to render the passage from partial to total reflection 

 more gradual, and thus to flatten the curve. To the same 

 cause may also probably be attributed the somewhat greater 



