film. It was also shown that at the top of the film, where the thickness was very small, a black band appears, its lower edge being sharply defined as though there were here a sudden change in thickness, as illustrated in Fig. 47.
Now, this "black spot" may be observed sufficiently long 
FIG. 47to measure the displacement produced in interference fringes when the film is placed in the interferometer. It is probable that over the area of the "black spot" the two surfaces of the film are as near together as possible; and if the water is made up of molecules, there are very few molecules in this thickness—possibly only two—so that a measurement of this thickness would give at least an upper limit to the distance between the molecules.
A soap solution of slightly different character from that used in the last lecture is more serviceable for this purpose.[1] With such a solution the film lasts a remarkably long time. It is interesting to note that some time after the "black spot" has formed, portions of its surface reflect even less light than the rest, and these portions gradually increase in size and number till the whole surface almost entirely vanishes.
It is found on placing such a film as this in the interferometer that there is no appreciable change in the fringes. The film is so thin that we cannot observe any displacement at all; if we place two films in the interferometer, the displacement should be twice as great; but even then it is inappreciable. To obtain a measurable displacement it was found necessary to use fifty such films. The arrangement
- ↑ This solution is made of caustic soda 1 gm., oleic acid 7 gm., dissolved in 600 c.c. of water.
