342 
REPORT —1854. 
small ; and when X is very great, or — extremely small, the value of p is 
X 
susceptible of a limit, which will be 
This will represent the physical condition, that as we take rays of successively 
greater wave-length, they will be crowded together into one position of re¬ 
fraction, which will have a bounding or limiting position, beyond which no 
ray of however great wave-length can be refracted. This will be different for 
each medium, but will in general correspond to a refractive index not greatly 
below the index for the extreme red ray, and which is calculated, in my trea¬ 
tise just referred to, for various media. For rock salt it is a little lower than 
Prof. Forbes’s index for dark heat. 
The data will be best seen as collected in the following Table*. 
Rock salt. 
Ray. 
Values of p. 
Obs. Forbes. 
Theory, PowelL 
1-558 
1*540 
1*529 
1*527 
\ —000079. 
Dark hot metal . 
Limit . 
1*528 
M. Knoblauch’s Researches. 
Among the most important of recent researches on the subject of radiant 
heat, are those of M. Knoblauch, Professor of Natural Philosophy in t e 
University of Marburg, which arc uot to be surpassed for elaborate esten 
and accuracy of detail. They are given in Poggendorff’s ‘Anualen, JW* 
and March 1847, and translated in Taylor’s ‘Foreign Scientific Memoirs, 
Parts XVIII. & XIX. 1848. 
The memoir is of great extent, and is divided into six sections. 
Section I. is entitled “On the Passage of Radiant Heat through Diatner- 
manous Bodies, with especial regard to the Temperature of the Source ot 
Heat" 
The author commences with a summary of the results previously obtained, 
in which he cites the results of Delaroclie and others, without reference to 
the different interpretation which must be put upou them if the experiments 
and conclusions just referred to be admitted. 
He observes, that, from the experiments of Melloni, rock salt appeal 
equally permeable by heating rays of all hinds : from those of Forbes, pre¬ 
pared rock salt would seem penetrated by heating rays in a greater degree 
when the source was at a lower temperature. Here 1 would observe that tae 
temperature of the source, as such, manifestly bears no direct proportion to 
the degreeof luminosity, it being perfectly well known that the temper™* 
ot luminosity is very different for different bodies; and these are also ot very 
lS^ofSecfpS pT/ r°S C “" er _ e S “i ,mitted t0 A. in 1840 and 
