of Metal Particles for Light- Waves. 263 



Ad attempt ^vas now made to determine whether the tihns 

 show anomalous dispersion as we shonUl expect them to do i£ 

 the colour is due to resonance. 



A sheet of thin plate-glass was prepared with gelatine and 

 flowed with the silver solution. A number of glass tubes 

 were then laid side by side on the plate, the fluid being drawn 

 up by capillarity into prisms. The plate was then warmed 

 until the prismatic strips of fiiiid had dried, when the glass 

 tubes were removed. On examining the plate it was found that 

 some very perfect prisms had formed, which resembled closely 

 the cyanine prisms which I have described in a previous paper. 

 The portion of the plate which seemed to carry the most 

 perfect prism was cut from the glass sheet and mounted on 

 the table of the spectrometer, a small portion of the glass 

 having been previously cleaned off to enable a reading to be 

 taken of the undeviated image of the slit. The prism was 

 screened with black paper to cut off the hght which did not 

 pass through the most perfect portion, and the slit of the 

 instrument illuminated with light from a monochromatic 

 illuminator. By alternately exposing the clear glass and 

 the prism, readings of the deviated and unde^dated images 

 could be taken. It was found that when blue-violet light 

 was employed the prism did not deviate the image by any 

 measurable amount, while with red light the deviation 

 amounted to several times the width of the image of the slit. 

 The latter was considerably broadened by diffraction, as is 

 always the case with strongly absorbing prisms. 



The prism was of a deep red colour, but transmitted violet 

 freely^the absorption-band being in the green. It was found 

 that the deviation was less for the extreme red of the spec- 

 trum than tor the orange-red, which is just what we should 

 expect. The absence of any deviation in the case of the 

 ^*iolet light means of course that the refractive index does 

 not differ much from unity for these waves. To determine 

 the index for the red it is necessary to know the angle of the 

 prism. This was determined by observing the images of the 

 slit reflected from the strip of clear glass and the surface of 

 the prism, the required angle being one-half of the observed 

 angle. This angle was found to be 1^ \b'' , while the de^na- 

 tion for red light was 2' -k'l' . The refractive index calculated 

 from these observations turns out to be 3' 15; or about that of 

 selenium for yellow light. 



This is practically proof positive that the cause of colour is 

 resonance, though the question is still open as to whether it 

 is resonance of small metal particles in a manner analogous 

 to the action of the tinfoil resonators on electro -magnetic 



