1880.J The Baconian Philosophy of Heat. 693 
temperature may, in reality, be any whatever), produce the 
effedts of heat on those bodies which they impinge. So 
much being granted, the further question arises, what is the 
relation between the undulations just described, and those 
other undulations which are now generally recognised 
(although Newton would not allow it) to constitute light in 
its state of propagation ? It had been found that the invisible 
radiations which produce but effects of heat were liable to 
exadtly the same vicissitudes as the visible radiations which, 
besides general effe(5ts of heat, produce also in our eyes the 
particular sensation denoted as light ; and, from a great 
discovery made by Sir W. Herschel, it was known also that 
among invisible rays there exist varieties of refrangibility 
entirely similar to those which Newton had discovered among 
the visible — limited, however, by this, that the most refran- 
gible of the invisible rays just equal in refrangibility 
the least refrangible of the visible rays. The natural 
inference from all this — which Herschel also at first had 
recognised, but from which, at a later period, owing to a 
strange misunderstanding, he dissented was stated at the 
beginning of this century by Dr. Young in the following 
words : — “ Suppose the undulations principally constituting 
[invisible] radiant heat to be larger than those of light ; 
while, at the same time, the smaller vibrations of light, and 
even the blackening [sometimes called photographic] rays, 
derived from still more minute vibrations, may, when suffix 
ciently concentrated, concur in producing the effects of heat. 
These effects, beginning at the blackening rays, which are 
invisible, are a little more perceptible in the violet, which 
still possess but a faint power of illumination; the yellow- 
green possess the most light ; the red give less light but 
much more heat ; while the still larger and less, frequent 
vibrations, which have no effedt on the sense of sight, may 
be supposed to give rise to the least refrangible rays,, and to 
constitute invisible [radiant] heat.” According to this view, 
invisible rays differ from visible intrinsically but in the same 
respedt as the blue, for instance, differ from the green rays — 
namely, in point of degree; although physiologically the 
difference both between visible and invisible rays, and be- 
tween blue and green rays, is immense. After considerable 
hesitation and contention, the opinion of Young has now 
become almost universally adopted — it being proved, on the 
one hand, that invisible and visible rays are alike in nature, 
and on the other, that the pretended separation of light from 
heat reposes in reality only on the separation of invisible and 
calorifically strong from visible and calorifically less poweiful 
