334 Proceedings of the 



looked as well almost as if cut ; and the pattern was so rich and 

 full, and of such a kind, as to preclude any imitation of it by 

 cutting. Mr. Pepys also placed a beautiful spiral metallic thermo- 

 meter, by Breguet, upon the table. 



NOTE BY M. F. 



In consequence of the necessity I was under of sending the paper 

 referred to in the above proceedings to press (page 205) by a 

 certain time, I was unable to pursue many of the beautiful com- 

 binations of form, colour, and appearance, to which the experi- 

 ments led, especially as they promised only amusement and little 

 more of instruction than the paper itself contained ; but one or 

 two varieties in the appearances, which have occurred to me since, 

 are so striking, that I am glad of the opportunity of noticing 

 them briefly in the same number with the paper. At page 218, 

 I have described the singular appearance produced when the 

 reflected image of a revolving cog-wheel, held before a glass, is 

 observed through the cog-wheel itself. If, in such a wheel, a little 

 nearer the centre, a series of regular apertures be cut, so as to re- 

 present cogs and their intervals, but the number different by 1.2.3, 

 or any small quantity, from the number of the cogs, then, upon 

 making the experiment as before, that series of cogs in the re- 

 volving wheel through which the eye looks will appear to stand 

 still, but the other series will travel in the spectrum : upon changing 

 the eye to the other series of apertures, then the quiescent part of 

 the spectrum will move, and the moving part become quiescent. 

 If two or three series more of such apertures be cut in the wheel, 

 concentric one to another, but the number of intervals varying in 

 each, then a great variety of changes are produced, as the eye looks 

 through one part or another of the wheel. The series of cogs in the 

 spectrum move with different velocities, or in opposite directions, 

 changing with the slightest motion of the eye. Two or three per- 

 sons looking through different parts of the wheel see appearances 



