Metal Particles for Light-Waves. 427 



colour. In continuing the work, I plan to make more ex- 

 haustive examinations with the microscope, using higher 

 powers if possible, employing photography, and ultra-violet 

 light i£ necessary, for I believe that only in this way can the 

 nature o£ the resonator be determined. There will be no 

 great difficulty in determining the dispersion, since the gold 

 films are permanent, and can be examined with the inter- 

 ferometer, or they may easily be given a prismatic form. I 

 feel confident that they will show anomalous dispersion, a 

 phenomenon which, if observed, would be almost proof 

 positive that the absorption-band was due to resonance. It 

 would not, however, enable us to decide whether the resonance 

 was within the molecule or not, for prisms built up of tinfoil 

 strips were found by Garbasso and Aschkinass * to refract 

 and disperse electromagnetic waves. Rubens and Nichols 

 have examined the action on heat-waves of resonator systems 

 formed by ruling crossed gratings on thin silver films, and 

 found evidences of a higher reflexion percentage when the 

 length of the resonators approaches a whole number of half 

 wave-lengths. 



Prof. Nichols and I are, at the present time, working in 

 collaboration on the selective reflexion from a number of 

 plates covered with resonators, very much smaller than any 

 that have hitherto been employed. By depositing thin films 

 of gold in vacuo, on glass, and ruling under oil, I have suc- 

 ceeded in producing resonators measuring 0*8 ^ by 1*1/*, 

 which should resonate in a part of the spectrum where there 

 is plenty of energy. Some of the trial rulings w r ere made on 

 the blue and purple films which I have mentioned, and in 

 examining them under the microscope I have detected 

 numerous minute granules, of about the same size as the 

 sodium particles which I described in my previous paper. 

 Whether these are the particles deposited by the cathode dis- 

 charge, or merely metallic dust cut off by the diamond point, I 

 am unable to say. The fact that the coloured films of gold are 

 produced when the glass plate is at some distance from the 

 diamond point, seems to indicate that the gold vapour, if we 

 may use the term, has time to condense into minute drops before 

 reaching the plate. These films adhere much less nrmly to 

 glass than the green films which are formed when the 

 distance between the cathode and the plate is small. In the 

 latter the deposit is doubtless molecular, and the optical 

 properties are similar to those of gold leaf. I have thus far 

 been unable to obtain coloured deposits of platinum, which 

 is the only other metal I have examined at the present time, 

 which fact makes it seem as if the molecule was in some 

 * Wiedemann's Annnlea, vol. liii. p. 534. 

 2 F 2 



