[162] PROFESSOR STOKES, ON THE 



of glass, lay in the centre of the system of rings. Supposing the image to have this position, 

 on moving the head sideways the opening out of the rings could be traced from its very com- 

 mencement. By moving back the head so as to keep the image of the luminous point in 

 the centre of the system of rings, it was easy to try the experiment to which Art. 13 relates, 

 the virtual image of the eye being thus kept in a line drawn through the luminous point 

 perpendicular to the mirror, and the eye moving relatively to the luminous point, which is as 

 good as if the luminous point had moved while the eye remained fixed. I found, in fact, that 

 on moving back the head the rings expanded till the bright central patch surrounding the 

 image filled the whole field of view, and on continuing to move back the head the rings 

 appeared again. In the position in which the central patch filled the whole field of view, 

 the least motion of the eye sideways was sufficient to bring into the field portions of excessively 

 broad coloured bands. 



Between the system of rings seen when the eye was respectively nearer to and further 

 from the mirror than when in the position in which the rings became infinite, there was one 

 difference which may here be mentioned. So long as the image occupied the centre of the 

 systems, they were similar to each other ; but when the head was moved sideways, the centre 

 of the circles passed in the first case to the side of the image towards which the head was 

 moved, and in the second case to the contrary side. This affords another way of comparing 

 with experiment the result of theory already mentioned relating to the direction of curvature, 

 and it will be readily seen that the result of experiment agrees with the prediction of theory. 

 For, suppose the distance of the eye from the oblique plate less than that of the luminous 

 point, so that the virtual image of the eye lies between the luminous point and the mirror, 

 and let the eye move to the right. Then its virtual image moves to the left, and therefore, 

 according to theory, the centre of curvature ought to fall to the left of the image of the 

 luminous point, right and left being estimated with reference to an eye supposed to occupy 

 the position of the virtual image of the actual eye. But on account of reflexion at the glass 

 plate, there is reversion from right to left, and therefore to the eye in its actual position the 

 centre of curvature falls to the right of the image of the luminous point, which agrees with 

 observation. The experiments described in this article may be tried very well with the 

 flame of a taper, but in examining what becomes of the rings when they expand it is more 

 satisfactory to use sun-light. 



16. In describing the disappearance of the rings, I said that the central spot expanded 

 till it filled the whole field. In truth, when the rings had expanded a faint luminous central 

 spot of finite size remained visible, which was surrounded by a dark ring, and then a faint 

 luminous ring. It would have been more correct to speak of the dark ring as faint, since 

 these rings consisted merely in slight alternations of intensity in a generally bright field. 

 These rings, however, had evidently nothing to do with the former rings, which had disap- 

 peared ; for they continued to have the image of the luminous point for their centre when 

 the head was moved to one side. They were doubtless of the same nature as those which are 

 seen when a luminous point, or the flame of a candle, is viewed through a piece of glass 

 powdered with lycopodium seed, and arose from the interference of pairs of streams of light 

 which passed on opposite sides of the globules of dried milk. I merely mention these rings 



