322 Dr. A. M. Mayer on a Method of detecting the Phases 



then the distance A to B^, B^ to B^, &e. will equal half wave- 

 lengths, while A to B^, B^ to B*, &c. will represent whole wave- 

 lengths (English) corresponding to the note given by A. If 

 another sonorous body, B, giving exactly the same note as A, 

 be placed anywhere on X, it will have vibrations communicated 

 to it from the vibrating air, almost as though its substance was 

 of the air itself. Now imagine this body B placed at B^, or at 

 B^, or B^, &c., then its phases of vibration will be exactly similar 

 to those of A; but when placed at B', B^, B^, &c., its phases of 

 vibration will be opposed to those of A. That is, at distances 

 from A equal to any number of whole wave-lengths the body B 

 will, at the same moment of time, swing with A, but at distances 

 from A equal to any number of half wave-lengths the direction 

 of its swings, at any given instant, will be opposed to A ; while 

 at intermediate positions, on the line X, the oscillations of B 

 will be lagging somewhat behind, or be slightly in advance of 

 the phase of A''s vibration. 



From this it is evident that if, by any means, we can see at the 

 same time the vibrations of A and of B, we shall (if the received 

 conception of the nature of a vibration's propagation is correct) 

 see their motions just as has been described above, and there- 

 fore be able to measure, directly in the air, a wave-length, and 

 to determine any wave-surface enclosing a vibrating body. I 

 have devised several processes. I will, however, here describe 

 two only ; the first, though impracticable, I speak of to render 

 clear the general method of all ; the second I give on account 

 of its simplicity, ease of execution, and the superior accuracy of 

 its numerical results. 



Take two tuning-forks giving the same note, and having mir- 

 rors attached to their similar prongs. Place one at A, the other 

 anywhere on the line X. Reflect a pencil of light from each 

 mirror of the forks on to a revolving mirror whose axis of rota- 

 tion is in a plane parallel to the planes of vibration of the forks. 

 If the fork B, which vibrates sympathetically, be placed at B^, 

 B^, B^, &c., then the two pencils reflected from the forks will, 

 on striking the revolving mirror, be drawn into two sinuous 

 curves, and the flexures of the two curves will run parallel with 

 each other; that is, the curves will appear as the two rails of a sinu- 

 ous railway ; but if the fork B be placed at B^, B^, or B^ &c., 

 then the sinuosities of the two curves will no longer be parallel, 

 but will be opposed to each other ; that is, where the flexure 

 of one of the curves is concave on the left, the correspondmg 

 flexure of the other curve will have its concavity on the right. 

 If the fork B be placed at intermediate positions in reference to 

 those above stated, we shall have neither concordance nor oppo- 

 sition of the flexures, but intermediate relations depending on the 



