402 MR. J. C. MAXWELL GARNETT ON 



that the minute spheres send no light directly up the tube when the electric vector 

 of the incident light is parallel to the microscope tube (Nicol perpendicular to plane of 

 incidence), less than half the green light from any small spheres will be cut off. The 

 cone of light will therefore have more green in proportion to the yellow-red than 

 before the introduction of the Nicol. Therefore the colour of the cone of light will be 

 more white than before (ii). 



When the Nicol is perpendicular to the plane of incidence, the green light from the 

 small spheres is cut off, so the colour of the cone of light will be more red than 

 with no Nicol, and therefore the total quantity of light sent up the tube will be 

 rather lessened (iii). The conclusions (i), (ii), (iii) are in accordance with the 

 phenomena tabulated in the 3rd, 4th, and 5th columns of Table II. 



The glasses Ca, Cb, Cc present no special difficulties. We have seen ( 3) that 

 those metal particles in a gold glass whose diameters are less than O'l/i (10~ 5 centim.) 

 are spherical, and (5) that small gold spheres send green light up the microscope 

 tube. In the above-named glasses the figures in the 7th column of Table II. show 

 that the particles are so small as to approximate to the spherical form. This is 

 confirmed by the green cone of light and its approximate extinction when the electric 

 vector of the incident light is in the direction of the microscope tube. 



As here, too, the observed particles are far enough apart to be distinguished under 

 the microscope, it is necessary to postulate additional minute spheres to explain the 

 pink colours of these glasses. 



In glasses D and E the blue and violet colours of the transmitted light present a 

 difficulty which I have not yet been enabled completely to surmount.* It is probable 

 that the particles in this glass are not sufficiently thickly distributed to satisfy the 

 condition of there being many particles to a wave-length of blue light. When the 

 incident light is blue, the absorption that we have investigated is therefore not 

 present. When, however, the incident light is red, there are sufficient particles to a 

 wave-length for absorption to take place. Thus, although if light of all wave-lengths 

 were absorbed, the red would be least absorbed ; yet here it is only the larger wave- 

 lengths that suffer the absorption whose nature we have investigated. 



PART II. 



7. With a view to examining whether these principles apply to the colour changes 

 exhibited by translucent films of metal when heated, observed by Mr. G. T. BEILBY 

 ('Roy. Soc. Proc.,' vol. 72, 1903, p. 226) and by Professor R. W. WOOD ('Phil. 

 Mag.,' vol. 3, 1902, p. 396), we proceed to consider the transmission of light by films of 

 metal, the metal being in the form of small spheres, many to a wave-length of light 

 in the film, 



* See Appendix added July 8th for explanation of Bine and Violet Colours. 





