2o 4 THE POPULAR SCIENCE MONTHLY. 



the mind was prepared to detect any resemblance presenting itself be- 

 tween the action of light and that of waves. Great classes of optical 

 phenomena accordingly appeared which could be accounted for in the 

 most complete and satisfactory manner by assuming them to be pro- 

 duced by waves, and which could not be otherwise accounted for. It is 

 because of its competence to explain all the phenomena of light that 

 the wave-theory now receives universal accejjtance on the part of 

 scientific men. 



Let me use an illustration. We infer from the flint implements 

 recently found in such profusion all over England and in other coun- 

 tries, that they were produced by men, and also that the pyramids of 

 Egypt were built by men, because, as far as our experience goes, noth- 

 ing but men could form such implements or build such pyramids. In 

 like manner, we infer from the phenomena of light the agency of waves, 

 because, as far as our experience goes, no other agency could produce 

 the phenomena. 



Thus, in a general way, I have given you the conception and the 

 grounds of the conception, which regards light as the product of wave- 

 motion ; but we must go further than this, and follow the conception 

 into some of its details. "We have all seen the waves of water, and we 

 know they are of different sizes different in length and different in 

 height. When, therefore, you are told that the atoms of the sun, and 

 of almost all other luminous bodies, vibrate at different rates, and pro- 

 duce waves of different sizes, your experience of water-Avaves will en- 

 able you to form a tolerably clear notion of what is meant. 



As observed above, we have never seen the light-waves, but we 

 judge of their presence, their position, and their magnitude, by their 

 effects. Their lengths have been thus determined, and found to vary 

 from about 3o ^ ffo th to - 60 ^ 00 th of an inch. 



But, besides those which produce light, the sun sends forth inces- 

 santly a multitude of waves which produce no light. The largest 

 waves which the sun sends forth are of this non-luminous character, 

 though they possess the highest heating power. A common sunbeam 

 contains waves of all kinds, but it is possible to sift or Jitter the beam 

 so as to intercept all its light, and to allow its obscure heat to pass un- 

 impeded. For substances have been discovered which, while intensely 

 opaque to the light-waves, are almost perfectly transparent to the 

 others. On the other hand, it is possible, by the choice of proper sub- 

 stances, to intercept, in a great degree, the pure heat-waves, and to 

 allow the pure light-waves free transmission. This last separation is, 

 however, not so perfect as the first. 



We shall learn presently how to detach the one class of waves from 

 the other class, and to prove that waves competent to light a fire, fuse 

 metal, or burn the hand like a hot solid, may exist in a perfectly dark 

 place. 



Supposing, then, that we withdraw, in the first instance, the large 



