104 REPORT—1863. 
duced, characterized by rays of greater refrangibility than the most refrangible 
among the incident Newtonic; so that, by this means, the converse of what 
may be called the second phenomenon of fluorescence—the transmutation of 
Newtonic rays into other but less refrangibleNewtonic rays—might be effected. 
6. Supposing that, by means of the experiment proposed, the transmutation 
of Herschellic into Newtonic rays, and of Newtonic into more refrangible 
Newtonic rays, had been successfully performed, there would still remain 
some difficulties, which, in the opinion of some perhaps, would mar the paral- 
lelism between the class of phenomena thus realized and those of ordinary 
fluorescence. As the most important of these differences the following may 
be mentioned :—According to Prof. Draper*, incandescent metals emit rays 
forming an unbroken spectrum, which, with the increase of temperature, ex- 
tends more and more through the Newtonic in the direction of the Ritteric 
compartment, whilst retaining all the rays previously emitted. But in fluo- 
rescence or phosphorescence, on the contrary, the spectrum of the rays emitted 
is very often broken in a manner perfectly characteristic of the substance by 
which they are emittedt. Again, in fluorescence, all the transmuted rays 
appear of less refrangibility than the active incident; but in incandescence, 
supposing it was produced by rays of a certain mean refrangibility, most 
probably both more and less refrangible rays than the incident would be 
found amongst those of the transmuted beam. The fact also that fluorescence 
may be excited by rays of comparatively small intensity, whilst the produc- 
tion of incandescence, in any case, requires rays of unusually great intensity, 
may appear asan objection ; but, in regard to this, (besides the doubtful com- 
parability of rays of different quality) it should be considered that the intensity 
of the active rays required to produce either phenomenon necessarily varies 
from substance to substance, according to the absorptive powers of each. 
Some further discrepancies of a similar nature to the last may yet be in- 
stanced. The production of incandescence by irradiation may possibly require 
time, or, so to speak, a repetition of the irradiation; its duration may be 
protracted beyond the time of incidence, and its extent not strictly confined 
to that of the actually irradiated spot. But fluorescence is instantaneous in 
its appearance and disappearance, as well as definite and limited in regard 
to extent. As, however, phosphorescence, which outlasts irradiation, seems 
now to be allowed as a variety of fluorescence, and the other two differ- 
ences, besides being of doubtful occurrence, also refer to questions of degree 
rather than of kind, perhaps not too much weight need be attached to them. 
7. But, whether incandescence produced by irradiationt and fluorescence be 
parallel phenomena or not, the production of the former, either by means of 
Herschellic rays only, or by Herschellic and slightly refrangible Newtonic rays 
(to be exceeded in refrangibility by the transmuted), in a manner analogous to 
ordinary fluorescent experiments, as described in this paper, cannot but de- 
serve a practical trial. The requirements for such an attempt, in the way of 
apparatus, consist principally of a large concave mirror, best of all of metal ; 
* Phil. Mag. vol. xxx. p. 349. 
+ Cf. (Prof. Stokes) Phil. Trans. 1852, p. 517; (M. E. Becquerel) Ann. de Chim. et de 
Phys. vol. lvii. tab. 2 (1859). 
t The term éncandescence is probably best employed as, in many respects at least, the 
counterpart of phosphorescence, both in its wider and in its more limited meaning. To 
designate the phenomenon which is the principal subject of the present paper, or the coun- 
terpart of fluorescence as defined in Art. 6, the term calcescence has been suggested to the 
author—from calciwm, the name of the characteristic chemical element of the substance 
whose action on the oxyhydrogen flame has first of all given rise to the speculations con- 
tained in this paper, 
