ON RADIANT HEAT. 
341 
various proportions, yet they left the point of maximum heat unaltered, 
i.e. though variously absorptive for the higher rays, they were not of a 
nature to stop the lower or most heating rays. 
It appears also to be questioned whether the solar beam does not actually 
contain some rays of Species I., that is, of wave-lengths greater than any of 
the prismatic rays, including the ordinarily invisible extreme red. My ori¬ 
ginal experiment on this point above referred to may probably be unworthy 
of comparison in accuracy with those which may now be made with the 
thermo-multiplier. 
It would therefore be highly desirable if an experiment on the same prin¬ 
ciple as mine, viz. a black and a white therraoseope exposed together, first 
with and then without a screen, were repeated ou the solar rays, with a 
variety of screens, including especially rock salt; with all the increased 
accuracy and sensibility now attainable by the use of two thermo multi¬ 
pliers , by which the differences or identities of ratio in the two c,;ses would 
be rendered evident in the most satisfactory manner. It would he also 
highly important to make similar observations on the oxy-bydrogen and the 
electric light. 
The theory of unequal wave-lengths, as the sole explanation of the dif¬ 
ferent spccic-s of “radiant heat,” whether solar or terrestrial, or in other 
words, the identity of the rays which produce alike the sensations of light, of 
heat, or other effects, each in some peculiar relation to the wave-length, cer¬ 
tainly applies in a very satisfactory manner to u largo portion of the pheno¬ 
mena. There may, indeed, be some minor objections or difficulties; but the 
only iormidable outstanding objection seems to arise from a single result, 
announced long ago by Melloni, and referred to in my former Report, viz. 
|he lact that a certain kind of green glass transmits the solar light in high 
intensity while it deprives it of all heating poioer. This anomaly is indeed in 
itself so singular as to require very positive authority to substantiate it; and 
in M. Melloni's statement there is, as appears to me, a certain degree of 
vagueness; and it is not supported by any numerical results, or even any 
detailed account of the mode of operating*. 
Au alleged isolated luct of so extraordinary a character has long appeared 
to nm to demand a strict re-examination: 1 had hoped that on presenting 
this Report, in many other respects so imperfect, I might have been able to 
announce the result of such a repetition. Rut, unhappily, a variety of causes 
have hitherto prevented me from carrying it into effect. 
Theoretical Hcfractkm of Heat. 
An important point bearing on the theory, was indicated very shortly after 
ic communication of my last Report (1840), in my treatise “ On the Undu- 
°7 Theory applied to Dispersion” (1S41). I have there shown (see 
PP- i t and 122) that the formula for the refractive index in terms of the wave- 
ength deduced from Cauchy's theory, furnishes a striking coincidence with 
in™ i° r | 8 ^termination of the index of refraction for a ray of dark heat 
rocA salt. The formula in question may be expressed thus,— 
fc p ~ q (x) ,+r (t)‘-* c - 
cule™ ^ a n le ' n ^ ex » ^ the wave-length, Ax the small interval of the mole- 
tj t ’ a ? . V ® constants. From the nature of this formula, it is evident 
as A is increased the changes corresponding in the value of /« are very 
On the Identity of Light and Heat, Taylor’s Foreign Memoirs, Part III. p. 388. 
