102 Prof. R. W. Wood on Optical Properties of 



is well known, transmit a very narrow region in the ultra- 

 violet (\ = 3000 to X==3200), but the transparency is not 

 very high if the film is thick enough to be quite opaque to 

 all visible light. 



Granular or Crystalline Films. 



We will now consider the remarkable optical properties of 

 the coloured films which are obtained when the metals are 

 distilled at room temperature. 



These were first studied by the author in 1901, and their 

 properties described in a series of three papers which appeared 

 in this Journal. 



At that time the provisional hypothesis was made that the 

 colour resulted from the electrical resonance of the minute 

 metal particles for light-waves. It appeared to be impossible 

 to explain the phenomena by any of the well known prin- 

 ciples of interference and diffraction, and the microscope 

 showed that the structure of the films was always granular, 

 the granules being usually so small that only their diffraction 

 disks appeared even under the highest possible magnification. 



Subsequent theoretical investigations by Garnett, Mie and 

 others showed that an optical resonance of the type suggested,. 

 i. e. analogous to the resonance of tinfoil strips to electro- 

 magnetic waves, was in all probability out of the question,, 

 and the colours were considered analogous to those exhibited 

 by colloidal mefals. 



As this explanation did not appear to me to be capable of 

 explaining all of the phenomena exhibited b}* the sodium and 

 potassium films, I made recently some further investigations 

 of the subject, which have some bearing, I think, not only 

 on the colours shown by certain insects and feathers, but 

 also, perhaps, on the colours which I found in the case of 

 granular films of collodion deposited on silver, and the 

 remarkable singularities of certain diffraction gratings which 

 I described a number of years ago, both of which cases have 

 been recently examined by Lord Rayleigh. 



Still more recently Prof. Mallock has described, in the 

 Proc. Roy. Soc, some experiments made with the coloured 

 films of oxide which form upon steel during the tempering 

 process. These colours are usually considered as the colours 

 of thin films, but it was found that a film of a given colour- 

 could be reduced in thickness by a polishing process without 

 altering the colour, even to the point of ihe complete 

 removal of the film, which of course caused the colour to» 

 vanish. 



