6i2 THE POPULAR SCIENCE MONTHLY. 



to tliis presently. The theories of light, however, involved matter 

 and motion. The corpuscular theory, as you well know, supposed 

 that excessively small particles were emitted from luminous bod- 

 ies, and traveled with enormous velocity. The undulatory theory, 

 which supplanted it, supposed that luminous bodies caused un- 

 dulations or vibrations in a highly tenuous matter called ether, 

 which is supposed to exist throughout the interplanetary spaces 

 and throughout the universe so far as we know it. Some suppose 

 this ether to be of a specific character, differing from that of ordi- 

 nary gases, others that it is in the nature of a highly attenuated 

 gas ; but, whatever it be, it can not be affected by undulations or 

 vibrations without being moved, and when matter is moved by 

 any force it must offer resistance to that force, and hence we get 

 antagonism between force and resistance. Light also takes time 

 in overcoming this resistance, i. e., in pushing aside the ether. It 

 travels no doubt at a good pace — about one hundred and ninety 

 thousand miles in a second ; but even at this rate, and without 

 being particular as to a few millions of miles, it takes three years 

 and a quarter to reach us from the star which, so far as we know, 

 is the nearest to us, viz., a Centauri. The ether, or whatever it 

 may be called, tenuous as it is, is not unimportant, though it be 

 not heavy. Without it we should have no light and possibly no 

 heat, and the consequences of its absence would be rather formi- 

 dable. I believe you have heard Dr. Tyndall on this subject. Sup- 

 posing the visible universe to be as it is now supposed to be, i. e., 

 in no part a mere vacuum, there can be no force without resist- 

 ance in any part of it. 



But photography carries us further, it shows us that light acts 

 on matter chemically, that it is capable of decomposing or forcing 

 asunder the constituents of chemical compounds, and is therefore 

 a force met by resistance. In the year 1856 I made some experi- 

 ments, published in the " Philosophical Magazine " for January, 

 1857, which seemed to me to carry still further what I may call 

 the molecular fight between light and chemical affinity, and among 

 them the following : Letters cut out of paper are placed between 

 two polished squares of glass with tin-foil on the outsides. It is 

 then electrized like a Ley den jar, for a few seconds, the glasses 

 separated, the letters blown off, and the inside of one of the glasses 

 covered with photographic collodion. This is then exposed to dif- 

 fuse daylight, and on being immersed in the nitrate of silver bath 

 the part which had been covered with the paper comes out dark, 

 the remainder of the plate being unaffected. (This result was 

 shown by the electric-light lantern.) In this case we see that 

 another imponderable force, electricity, invisibly affects the sur- 

 face of glass in such a way that it conveys to another substance of 

 definite thickness, viz., the prepared collodion, a change in the 



