Prof. Tyndall on Vibrai ions produced by an Eledric Current, 417 



omitted. For the purpose of reversing the position of the lettei-s 

 which appear placed like type, a prism-stereoscope may, once 

 for all, be added to a stereoscope of the above description. If 

 it be required to test the identity of two sentences consisting of 

 the same words set up in type, or of two coppei--plates before 

 printing, the stereoscope above described is applied at once to 

 the plates, which are laid side by side. In applying the mirror- 

 stereoscope, it is advisable to add two magnifying glasses. As, 

 in this manner, means are afforded to the physicist of testing the 

 equality of the distances between lines, so also the circumstance 

 that in a well-forged bank note the most striking differences 

 immediately show themselves may be cited to justify the appli- 

 cation of the stereoscope to the detection of false paper money. 

 It will then be advisable for the Government, if such a note be 

 discovered to be forged, to publish the result of the stereoscopic 

 analysis as an infalhble means of detection. If several plates 

 are employed in the preparation of the genuine paper, these 

 must of course be treated as distinct originals. A copy which 

 only presents the slight difference which results from the un- 

 equal expansion of the paper, can by the above method be di- 

 stinguished from one absolutely identical. 



LXX. On Vibrations produced by an Electric Current. 

 By John Tyndall, F.R.S. 



THE last Number of the Philosophical Magazine contains an 

 interesting note from Prof. Forbes on cei'taiu Vibrations 

 produced by Electricity. Without knowing that Prof. Forbes 

 had written anything upon the subject, I had in a former Num- 

 ber of the Philosophical Magazine appended a short note to a 

 paper by Mr. Gore, in which I had referred the rotation observed 

 in his experiment to the heat produced by the electricity. Prof. 

 Forbes, if I understand him aright, considers the action due to 

 a repulsive power possessed by the electric current itself, and 

 considers that a similar repulsive power, on the part of a current 

 of heat, explains the Trevelyan experiment. 



Were this the case, I think it will be agreed that a point 

 altogether new, and of very great importance regarding both 

 heat and electricity, would be established. But, as far as my 

 experiments go, they do not countenance this notion. Many 

 years ago. Page excited Trevclyan's sounds by electricity ; and it 

 is now, 1 think, nearly six years since I made the experiment 

 myself before the audience of the Royal Institution. I then 

 found tliat, by continuing the action with the voltaic current for 

 a sufficient time, the vibrations were continued even after the 



Phil. May. S. i. Vol. 17. No. 110. .lune 1859. 2 F 



