[172] PROFESSOR STOKES, ON THE 



• 31. Having cleaned the surface of a concave mirror which had been employed in forming 

 the bands, I rubbed a little tallow on it, and then wiped the mirror in one direction with 

 a handkerchief, so as to leave a finely striated tarnish on it. The tarnish was not suffi- 

 cient to cause much obscurity ; but the image of a candle seen in the mirror was accom- 

 panied, as is usual in such cases, by two tails of light, which ran out in a direction 

 perpendicular to the strias. Having placed a small flame in the centre of curvature of the 

 mirror, I found that the rings were formed with great brilliancy where they were inter- 

 sected by the tails of light, but elsewhere they were almost wholly invisible. 



Now the tails of light are known to be a phenomenon of diffraction : the striated tarnish 

 may in fact be regarded as a sort of irregular grating, and the tails of light are of the 

 nature of Fraunhofer's spectra. If a tarnish in general were capable of producing rings 

 independently of diffraction, there appears no reason why a tarnish of tallow should not be 

 capable ; for the particles of tallow are as fine as those of most other kinds of tarnish. But if 

 in the case of a tarnish of tallow the deflection of the light from its regular course be not a 

 phenomenon of diffraction, there appears no reason why the rings should be confined to the 

 tails of light in the experiment described above. 



32. The phenomena of polarization seem however to lead to a crucial experiment for 

 deciding whether the deflection of the light from its regular course, which enables the rings to 

 be formed, be a phenomenon of diffraction, or of scattering in the strict sense of the term. 

 When polarized light is scattered, as for example when it is reflected from or transmitted 

 through white paper, it loses its polarization, but when polarized light suffers regular diffrac- 

 tion it retains its polarization. 



Having placed a small flame near the centre of curvature of a concave mirror, of which 

 the surface had been prepared with milk and water, I placed a Nicol's prism close to the 

 flame, so as to polarize the light incident on the mirror. On examining the rings with another 

 Nicol's prism, they proved to be perfectly polarized. 



33. It may not be considered out of place here to point out what appears to be the cause 

 of a phenomenon observed by M. Pouillet. In an experiment in which rings were occasioned 

 simply by the straight edge of an opaque body held in front of a metallic speculum, it was 

 found that they were formed distinctly in only one half of their circumference. The reason 

 of this appears to be simply as follows. As the waves of light pass the diffracting edge in 

 their progress towards the mirror, those rays which are diffracted inwards, so as to enter the 

 geometrical shadow, after being regularly reflected at the mirror fall upon the opaque body, 

 by which they are stopped. As these rays are required for the formation of that half of the 

 system which lies on the same side as the opaque body, the other half only is well formed. 

 The first half may be formed obscurely by a few rays which are diffracted in the required 

 direction at such a distance from the edge that on their return they pass clear of the edge, 

 and so proceed to interfere with other rays diffracted by the edge on the return of the general 

 wave. 



