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VII. Optical Properties of Homogeneous and Granular Films 

 of Sodium and Potassium. By R. W. TVood, Professor 

 of Experimental Physics, Johns Hojilins University *. 



[Plate I.] 



IN the Philosophical Magazine for 1902 I described some 

 observations made on films of the alkali metals deposited 

 by distillation in exhausted glass bulbs. 



These films showed brilliant colours both by transmitted 

 and reflected light, and as the microscope showed that the 

 films were made up of minute metallic granules almost sub- 

 microscopic in size, the provisional hypothesis was made 

 that the colours were due to an optical resonance, analogous 

 to the electrical resonance exhibited by small strips of tin- 

 foil mounted on a glass plate. 



Subsequent theoretical investigations by Grarnett and by 

 Mie showed that this hypothesis was quite untenable, thougli 

 their treatment of the subject does not preclude the existence 

 of a molecular resonance* 



There appears to me to be a large number of cases in 

 which brilliant colours are shown, which cannot be explained 

 by any of the common laws of optics with which we are 

 familiar. The case of the frilled films of collodion on a 

 reflecting surface of silver, to which I called attention many 

 years ago (Phil. Mag. April 1904), and which has recently 

 been examined by Lord Rayleigh, is a case in point. Still 

 more recently Mallock has raised the question as to the cause 

 of the colours shown on the surface of tempered steel, which 

 are usually considered as thin film effects due to oxidation. 

 His paper, published in a recent number of the 'Proceedings 

 of the Royal Society,' shows that the sequence of the colours 

 is similar to that exhibited by a periodic structure, such as a 

 fine-meshed net mounted on a silvered glass mirror. It 

 seems to me, however, to be not very clear how colour can 

 be produced in the case of the tempered steel by the same 

 means, owing to the extreme thinness of the film. In some 

 respects the case seems analogous to that of the frilled 

 collodion films previously alluded to. 



So far as I am aware no very satisfactory explanation has 

 ever been given of the colours of certain feathers and butter- 

 flies, and I strongly suspect that there is some action of 

 absorbing matter in a state of very fine division, upon light- 

 waves, which is not yet completely understood. In the 



* Communicated by the Author. 



