320 Prof. R. W. Wood on the 



electrodes the half-silvered films of a Fabry and Perot inter- 

 ferometer. These can be adjusted to absolute parallelism by 

 optical methods, and the exact distance between the metallic 

 surfaces can be determined by placing a white light behind 

 them and counting the number of interference bands which 

 appear in the spectrum of the transmitted light between two 

 lines of known wave-length. The interferometer plates were 

 half-silvered by the formaldehyde process, which gives the 

 most uniform semitrausparent films, and were adjusted to 

 parallelism with the soda flame, circular fringes appearing 

 by transmitted light, the diameter of the central circle re- 

 maining fixed as the eye is moved about. The plates were 

 mounted in the instrument a little to one side of one another, 

 so that the silver-leaf electrodes could be brought into light 

 contact with the silver films. One dry cell was used with a 

 milliammeter and the plates were gradually approached. 

 In every case a sudden large deflexion of the milliammeter 

 showed that metallic contact had occurred when the plates 

 were still separated by a distance amounting to thirty or 

 forty wave-lengths. It is apparently impossible to obtain 

 silver films free from projecting metallic points, which bridge 

 over the gap which separates them. I imagine that minute 

 matter in suspension or particles of reduced silver settle on 

 the film, and are not removed by the washing process. It 

 seemed, however, as if a cathode deposit of gold thrown 

 down in a vacuum would be free from these objections. 

 Semitransparent films of beautiful quality were deposited on 

 ■the interferometer plates and the experiment repeated. The 

 same thing happened as before. After repeated trials, using 

 great precautions to remove all dust particles from the glass 

 surfaces, I finally obtained a pair of films which gave a small 

 deflexion of the milliammeter when separated by a distance 

 of 28 wave-lengths. The fringes were still circular, or rather 

 the entire surface of the interferometer plate appeared 

 uniformly illuminated. On gradually approaching the plates 

 the current increased with beautiful regularity, but suddenly 

 when the plates were still over 15 wave-lengths apart a 

 large deflexion was suddenly obtained, showing that metallic 

 contact had occurred. As the plates were still further ap- 

 proached the fringes straightened out and crowded into the 

 field, showing that there was something between the plates 

 which caused a departure from parallelism. I have made 

 repeated trials with gilded plates, but on only two occasions 

 have I succeeded in getting the true effect before metallic 

 contact occurred at some point. It is impossible to polish 

 the films without making them much worse than before. 



