THE LAWS OF FILMS. 627 



is left behind, which will be yellow. Color is a partial extinction 

 of light not of light as a whole, but a suppression of one of its 

 constituents. If you take a yellow glass and allow the light to 

 fall through it, you will find it transparent ; in the same way a 

 blue glass is transparent ; but if these glasses are the comple- 

 mentary blue and yellow color, and placed one on top of the other, 

 no light comes through them. The yellow glass sifts out all the 

 blue rays, and the blue glass sifts out all the other rays, and no 

 light can get through. If the colors are not pure, it is usually 

 because the yellow has some green in it, and so has the blue. 

 Neither the yellow glass nor the blue is competent to sift out 

 these rays, so we see green come through them both. This is the 

 case in mixing blue and yellow in paints: the resulting green 

 does not come from the mixture, but is the sediment you might 

 almost call it left after the pure blue and pure yellow have neu- 

 tralized each other. 



It is clear that, if two waves can be made to set into vibration 

 the same medium at the same time, and from almost exactly the 

 same center, one of them being a half-wave or several half- 

 waves' length behind the other, we shall have, as in the case of 

 water and sound, no movement, or darkness. If there is not 

 exactly a half-wave's distance between them, some color-waves 

 will neutralize each other and be extinguished, and we shall get 

 the complementary color the resultant of all that is left un- 

 neutralized. 



This is the cause of all the flitting and changing colors in soap- 

 bubbles, mother-of-pearl, peacocks' plumage, opals, and iridescent 

 glass. By some means certain vibrations have been extinguished 

 by interference, and we see the resultant of the rest. Whenever 

 light goes from one medium into another, even when both media 

 seem perfectly transparent, there is a partial reflection from the 

 surface where the media meet. Hold a pin against the surface of 

 a piece of glass (unsilvered plate glass is the best) : you will see two 

 faint reflections of the pin, one from the front surface of the glass 

 and one from the back, and yet the main part of the light reflected 

 from the pin goes through, as you can easily tell by looking 

 through the glass at the pin. So it is with a soap film : when light 

 falls on it, most of it goes through, but there is a slight reflection 

 from the outer surface of the thin lamina of soap-suds and another 

 slight reflection from the back of it. The two sets of reflected 

 waves start from points so very near each other that they both 

 act on the medium in different directions at the same time and in 

 the same place, and we have color. 



If light went forward like a regiment of soldiers in line, there 

 might be just as much interference from the plate of glass as 

 there is from the film of soap-suds ; but it does not it goes out in 



