406 Prof. R. W. Wood on the 



metrical, it is clear that a considerable error may be introduced 

 in this way. 



I For a direct experimental test we require some means of 



cutting down the amplitude without introducing a retardation 

 or change of phase. In searching for a possible substance 

 which would absorb but not retard, I tried films of smoke on 

 glass. It seemed possible that, since the absorption is caused 

 by very minute opaque particles of carbon, the wave might 

 be transmitted without retardation. If this were found to be 

 the case a wedge-shaped film of smoke would furnish the 

 necessary conditions. Examination of the films by means of 

 the interferometer showed, however, that a very marked 

 retardation was introduced. This phenomenon I subsequently 

 found had been previously observed by Rosicky * and Stark f. 

 Stark's results indicated that in the case of films deposited 

 by coal-gas flames, we are dealing with a porous mass con- 

 sisting of 228 per cent, of pure carbon, and 97*72 per cent, 

 of air. He considered the case as that of a turbid medium 

 made up of air with a low, and carbon with a high refractive 

 index, such mixtures being known to possess a refractive 

 index intermediate between the indices of the constituent 

 parts. He made no determinations of the dispersion. 



It occurred to ms that the retardation in this case might 

 be ascribed to diffraction, there being an increase of path 

 due to the passage of the light waves around the carbon 

 particles, the case being analogous to the pnssage of a sound- 

 wave through a medium containing obstacles symmetrically 

 distributed. I succeeded in photographing the retardation of 

 a sound-wave resulting from its passage through several 

 layers of glass tubes, placed side by side with spaces between. 

 If the retardation of the light by carbon films is due to 

 diffraction, we should expect either to find dispersion absent, 

 which would mean that the path increase is the same for all 

 waves, or else a greater retardation for the long waves than 

 for the short. 



An attempt w r as at once made to determine the presence or 

 absence of dispersion, the experiments showing conclusively 

 that the long waves (red) were retarded more than the short 

 ones (violet), the condition found within the absorption-band 

 in the case of substances showing anomalous dispersion. 

 Experiments were made with smoke-films, and with films 

 deposited in a vacuum on plate-glass by the filament of an 

 incandescent lamp. The average diameter of the carbon 



* Rosickv, Sitzb. Ber. Wien. Ah. 1878. 

 t Stark, VVied. Ann. lxii. p. 351. 



