of Light by Coloured Media. 407 



tuning forks of the same pitch, and heating the ends of them, 

 fasten with sealing-wax, on one of them one, and on the other 

 two, disks of card, (all equal in size,) on the inner surfaces, 

 having the plane of the card perpendicular to that of a section 

 of the fork through the axes of both its branches. The cards 

 on that fork which has two, should have their surfaces about 

 a tenth of an inch asunder, and their centres just opposite ; 

 and the other fork should be brought into unison with it by 

 loading its undisked branch with additional wax, equal in 

 weight to the disk and wax on the other. Now strike the 

 forks, and a remarkable difference will be perceived in the in- 

 tensity of their sounds. The fork with one disk will utter a 

 clear and loud sound, while that of the other will be dull and 

 stifled, and hardly audible, unless held close to the ear. The 

 reason of this difference is that the opposite branches of the 

 fork are always in opposite states of motion, and that in conse- 

 quence the air is agitated by either the two branches vibrating 

 freely, or by both loaded with equal disks, with nearly equal 

 and opposite impulses; whereas in the case of a fork furnished 

 with only one disk, a greater command of the ambient medium 

 is given to the branch carrying it, and a much larger portion 

 of uncounteracted motion is propagated into the air. Here 

 then we have a case in which a vibrating system in full activity 

 is rendered, by a peculiarity of structure, incapable of sending 

 forth its undulations with effect into the surrounding medium ; 

 while the very same mass of matter, vibrating with the same 

 intensity, but more favourably disposed as to the arrangement 

 of its parts, labours under no such disability. 



The disked tuning fork is a most instructive instrument, 

 and I shall not quit it until I have availed myself of its pro- 

 perties to exemplify the easy propagation of vibrations, of 

 a definite pitch, through a system comparatively much less dis- 

 posed to transmit those of any other pitch. Take two or more 

 forks in unison, and furnish each of them with a single disk 

 of the size of a large wafer, looking outwards. (See fig. 17., 

 Art. 186. of my Essay on Sound, for the mode of attaching 

 such a disk.) Having struck one of them, let its disk be 

 brought near to that of the other, centre opposite to centre, 

 and it will immediately set the other in vibration, as will be 

 evident by the sound produced by it when the first fork is 

 stopped, as well as by its tremors, sensible to the hand which 

 holds it. The communication of the vibration is much more 

 powerful and complete when a small loop of fine silver wire is 

 fixed to one of the forks, and brought lightly into contact with 

 the other, with its looped or convex side. Imagine now a se- 



