428 Prof. R. W. Wood on tlie Electrical Resonance of 



way concerned with the colour. The variable nature of the 

 colour, however, especially in the case of sodium and potassium, 

 makes it appear improbable that the action is similar to that 

 of aniline dyes ; namely, a resonance within the molecule ; 

 for deposits of the same substance can be obtained showing 

 an absorption-band almost anywhere in the visible spectrum, 

 which is not the case with the more common types of coloured 

 media. 



The colour of the gold deposit varies, as I have said, with 

 the conditions under which the deposition takes place. I 

 have employed gold cathodes of two forms : a flat plate 

 about 3 cms. square, and a thick wire, screening off the 

 radiation from all but the tip with a mica screen. The most 

 interesting deposits were obtained from the small source. 

 In one instance the film showed a brilliant green surface 

 colour, resembling fuchsine, the transmitted light having a 

 purple tint. Owing to the transparency of the film a good 

 deal of white light is mixed with the selectively reflected 

 light ; this can be cut off with a nicol, if the reflexion takes 

 place at the polarizing angle for glass, and the coloured light 

 from the film which is unpolarized then appears in great 

 purity. One plate showed patches of brilliant carmine red, 

 deep blue, and green, of an intensity and saturation which I 

 have never seen equalled in any interference experiment. 

 The colour of the selectively reflected light depended some- 

 what on the angle of incidence, a phenomenon observed also 

 in the case of the sodium and potassium films. Increasing 

 the angle of incidence changed the colour from grcn to blue; 

 the period of vibration of the resonator system appears there- 

 fore to be less when the angle of incidence is large. 



If the glass plate is placed n^ar the tip of the gold wire, 

 the green deposit, similar to gold leaf in its optical propert es, 

 is deposited at the centre. Surrounding this is a film appear- 

 ing light yellow by transmitted light, and bluish by reflected 

 light. This seems to be what we should expect, for the 

 smallest particles, which will resonate for blue light, will be 

 deposited when the distance from the cathode is a little 

 greater than that at which the molecular deposit occurs. 

 Increasing the distance, w e get larger particles, and the point 

 of maximum resonance mo\es up into the green, giving us a 

 purple film with green surface colour. At a still greater 

 distance we get particles large enough to resonate for red, 

 and the film appears deep blue by transmitted light. All of 

 these variously coloured films can be changed into the green 

 structureless film by heating. We may regard the change 

 as due to the fusing together of the resonators. If this is 

 £he case, the electrical resistance should be less after firing 



