626 THE POPULAR SCIENCE MONTHLY. 



iar fact in regard to films their wonderful and changing colors, 

 what in scientific language is called " the colors of thin plates/' 

 because the same effect is produced in many cases where " film " 

 is not exactly the word to use. 



The colors of soap-bubbles furnish one of the most triumph- 

 ant vindications of the wave theory of light, and offer to its 

 opposers one of the hardest possible nuts to crack. A brief expla- 

 nation of the wave theory of light and interference, so far as it 

 bears upon our subject, will, it is hoped, be pardoned. It is an idea 

 so familiar to those who have studied physics, and yet so difficult 

 of conception to those who have not, that a few words seem neces- 

 sary in a popular exposition of the colors of films. 



Light is, of course, our name for the sensation, but back of the 

 sensation there lie the physical conditions which are its cause. 

 The theory of Newton, that light is caused by minute particles 

 of matter shot out from the luminous body, stood the test of the 

 simpler phenomena ; but, when it came to the explanation of soap- 

 bubble colors, his theory, even with the marvelous ingenuity 

 which he brought to bear upon it, broke down. If light were 

 matter, it is impossible to see how one light can be added to 

 another light and produce darkness, which is sometimes the case-, 

 while, if it were motion, we can readily see how motion may be 

 added to motion, and the result be rest. A sound can be so added 

 to a sound as to produce silence, but the more familiar illustration 

 is with waves of water. Two stones dropped into water will pro- 

 duce waves, and where these meet there are points at which the 

 water remains at its original level. This is because at these points 

 one set of waves tends to raise the water while the other set 

 tends to lower it, and between the two it remains where it origi- 

 nally was. This occurs in some parts of the ocean where the tidal 

 wave sweeps around an island and meets, one wave being half a 

 length behind the other, in which case they simply neutralize 

 each other. Where the crest of one wave would have been, the 

 trough of the other would have been at the same time, and be- 

 tween the two impulses in opposite direction at the same moment 

 the water remains unmoved, and there are no tides. 



Darkness corresponds with this unmoved plane of water, and 

 with silence in the case of sound-waves. If light were simple 

 waves, as a result of such interference we would simply have 

 darkness, and as a result of partial interference we would have 

 all the gradations from darkness to light ; but a light-wave is not 

 a simple undulation, it is made up of innumerable vibrations of 

 various wave-lengths, each of which corresponds with a color or 

 tone. The resultant of all these motions combined is white light. 

 Extinguish one rate of vibration, say the smaller waves which cause 

 the sensation of blue, and we have a wave the resultant of all that 



