426 Prof. R. W. Wood on the Electrical Resonance of 



depends to a certain extent on the dielectric constant of the 

 medium in which the dye is dissolved. 



Continued investigation along the lines indicated in my 

 previous paper has convinced me that it is impossible to 

 refer the colours either to interference or diffraction, and it 

 remains only to determine whether the resonance of the metal 

 films is molecular, as in the case of the aniline dyes, or 

 whether we are dealing with an electrical vibration of metallic 

 masses, smaller than the light-waves though of the same 

 order of magnitude. At the time of writing my first paper 

 I had been successful in producing coloured films only with 

 sodium, potassium, and lithium. Consequently, the only 

 optical investigations that could be made were such as could 

 be adapted to films formed on the walls of exhausted bulbs. 

 Determinations of dispersion were practically impossible 

 under these conditions, except by means of very elaborate 

 apparatus, which it did not seem worth while to construct 

 until further efforts had been made to obtain films of this 

 nature which would be permanent in air, and could be 

 examined with the interferometer. I have since succeeded 

 in producing deposits of gold, which exhibit colours very 

 similar to those shown by the potassium films. The deposit is 

 formed by the discharge from a gold cathode in a moderately 

 high vacuum, the colour depending on the distance of the 

 glass plate from the electrode and the degree of exhaustion. 

 Under certain conditions a green film is deposited, of a tint 

 similar to that of gold leaf, while under other conditions 

 blue, violet, and purple films can be obtained. The colour of 

 the green film is doubtless to be ascribed to the same causes 

 which operate in the case of gold leaf. The tint of the blue 

 and purple films, however, is changed by moistening the plate 

 with ligroin, precisely as is the case with potassium deposits. 

 The mere approach of a glass rod, moistened with the liquid, 

 is sufficient to produce a change of colour, the film apparently 

 possessing the power of condensing the vapour upon its sur- 

 face. Platinum and other metals in a fine state of division 

 have this same property, consequently it seems extremely 

 probable that the gold deposit is of this nature. Thus far 

 I have only obtained two or three deposits which show this 

 change. Most of the films are but slightly, if at all, affected 

 by ligroin. 



In the case of the gold films, the particles are too small to 

 be seen under the microscope, with the facilities at my dis- 

 posal ; and I am inclined to the opinion that, in the case of 

 the sodium and potassium films, particles which were actually 

 seen with the microscope were only the moderately large 

 ones, and may not have been instrumental in producing the 



