227 



wliilc nearly all the thermic rays are transmitted, constituting 

 what has been called ^' dark or " invisible heat. Tliat 

 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 terra '^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 raj^s which have that power, 

 by the action of certain substances on which the}^ 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 f)e 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 



