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while nearly all the thermic rays are transmitted, constituting 
what has been called “dark” or “invisible” heat. That 
concentrated, though invisible, heat really exists at the focal 
point may be readily shown by employing it in lighting a match 
or a cigar, and if a thin sheet of platinum coated with a deposit 
of the same metal in a state of minute subdivision, in order to 
render it more absorbent of heat, be so placed as to receive the 
focal rays, it will immediately become white hot, and a visible 
image of the carbon points will be produced on its surface. 
Professor Tyndall inferred that as these rays were invisible, 
that is, that they were incapable of affecting the retina of the 
eye, they would produce no effect on that structure, however 
concentrated; he therefore so placed his own eye that the focus 
might fall on his retina, and perceived no effect whatever ; the 
vibratory motion was there in all its intensity, but there was 
no heat , because the appropriate means of perception were 
absent. But on the contrary if the skin of the hand were 
placed at this focal point, it would speedily become charred, 
thus showing its power of being affected by heat. 
28. The term “invisible light” has been made use of ; but in 
reference to the definition given above, it evidently involves a 
contradiction; the term has been applied to those rays which 
are incapable of affecting the eye, but are at the same time 
capable of being changed into other rays which have that power, 
by the action of certain substances on which they may fall. 
29. Light and heat have frequently been illogically designated 
simply as “modes of motion” by able physicists; this appears 
to have led many (the authors of the above-mentioned essays 
not excepted) into a hopeless confusion of the terms force, 
energy, and motion. Doubtless in common parlance the terms 
light and heat will continue to be applied not to- the sensuous 
impressions produced, but to the agent producing them ; but it 
must be borne in mind that they are forms or kinds of energy, 
and not “ modes of motion.” 
30. It may be remarked that light and heat, electricity and 
magnetism, which are all now more or less generally recognized 
as forms of energy, have all been assumed to be material , but 
imponderable. The Newtonian or corpuscular theory of light 
sufficed to explain ordinary optical phenomena until the dis- 
covery of diffraction and interference, when a very forced 
supplementary hypothesis became necessary — namely, that the 
molecules of light were egg or spindle-shaped, and made 
perpetual somersaults during their onward progress, rebounding 
or being reflected from the surface of a medium, if they en- 
counter it sideways, but penetrating and being refracted, if they 
meet the surface endwise : but even this is insufficient to 
