352 Prof. J. J . Thomson : Experiments on 



so nearly equal to that of glass that no appreciable effect is 

 produced by the film of that substance between the face of 

 the prism and the unruled surface of the grating. 



On looking through the face of the prism which was per- 

 pendicular to that through which the incident light entered, 

 a white image of the slit was seen, together with diffraction 

 spectra on either side of it. 



Another experiment was performed, using a grating ruled 

 on glass with 300 lines to the inch. The width of a ruling- 

 was very small in comparison with that of a space. The 

 grating was of a cheap kind, and the spacing was probably 

 not very regular ; nevertheless phenomena similar to those 

 described above were observed. When the light was inci- 

 dent at an angle of 45° on the grating only a few spectra, of 

 high orders, were seen. 



When a strip of a screen such as is used for half-tone 

 process work was used as a grating the number of lines to 

 the inch being equal to 135, no diffraction spectra were seen 

 when the angle of incidence was equal to 45°. 



The probable existence of the diffraction phenomena de- 

 scribed above was, in the first place, deduced by one of us 

 from theoretical considerations; subsequently the experiments 

 described were devised and executed. The singularity of the 

 results obtained entirely vanishes when the point of view 

 chosen is that of the wave theory of light. On the other 

 hand, the failure of light to penetrate a free surface, com- 

 bined with the readiness with which it traverses the same 

 surface when parts are rendered opaque, afford sufficient 

 interest, we hope, to merit this short notice. At an earlier 

 date, when the wave theory was in more need of confirmation 

 than at present, the experiments described might possibly 

 have appeared as of a fairly crucial nature. At present they 

 may at least serve to illustrate, in a striking manner, certain 

 important points in the wave theory of light. 



XXXIX. Experiments on Induced-Radioactivity in Air, and 

 on the Electrical Conductivity produced in Gases when they 

 pass through Water. By J. J. Thomson, M.A., F.R.S., 

 Cavendish Professor of Experimental JPhysics, Cambridge*. 



IT has been shown by Elster and Geitel f that a wire, if 

 strongly negatively electrified for several hours either 

 in the open air or, as in one of their experiments, in a large 



* Communicated by the Author, 

 t Physikalische Zeitschr. ii. p. 563. 



