M. Terquem on the Vibroscope Tonometer. 213 



upon the terminal section of one of the branches of the auxiliary 

 diapason. The facets of this crystalline powder, obliquely illu- 

 minated by means of a lamp and lenses, form luminous points 

 of great brilliancy and extreme delicacy. The cursor of the 

 standard having been fixed upon the first stroke of the division, 

 the auxiliary diapason is put into exact unison with the former 

 by shifting the cursors, finally putting on them small pieces of 

 wax. The operator is guided in this operation evidently by the 

 transformations of the elliptic curve which is due to the coexist- 

 ence of the vibrations; and he stops when it diuiinishes gradu- 

 ally and very slowly without changing its form. 



This done, he moves the cursor of the standard diapason so as 

 to obtain about one beat per second, and determines the exact du- 

 ration of at least fifty beats by aid of a pointing telltale, observing 

 the oscillations of the elliptic curve. If the conditions be good, the 

 total duration of fifty beats scarcely varies a half second in several 

 successive determinations, which gives an approximation within 

 tJtt of a second for one beat, and consequently permits the num- 

 ber of vibrations to be determined within less than 0*01 . The 

 cursor of the auxiliary diapason is then shifted till unison is re- 

 established, which is ascertained by the fixity of the curve pro- 

 duced. The cursor of the standard diapason is then placed on 

 the second stroke of the division, and the duration of the beats 

 produced determined again. 



The operation is continued in the same manner by shifting 

 successively the cursors of the two diapasons until a sound is 

 obtained sufficiently distant from ut 2 , such as mi 2 , capable of 

 giving with the sound ut 2 a sufficiently simple acoustic curve 

 (4:5). When, by successive stops, the auxiliary diapason is 

 put in unison with the sound mi 2 , the cursor of the first standard 

 diapason must be carried back to the first stroke, and it must be 

 ascertained if the sound mi 2 is perfectly true ; if there is any 

 difference, it is determined by the duration of the oscillations of 

 the acoustic curve. The sum total of the numbers of beats per 

 second (the inverse of their duration) which have been observed 

 on the shifting of the cursors in succession, gives the difference 

 of the numbers of vibrations of the sounds ut 2 and mi 2 , of which 

 we have the ratio — which agrees with the principle given by 

 Scheibler for the construction of the tonometer, and permits the 

 absolute number of the vibrations of ut 2 to be calculated. 



Continuing in the same manner, we arrive at ut 3 . On the 

 way numerous datum- and verification-points are met with m 

 the simple intervals, such as the fourth, the fifth, the sixth, &c. 

 At length we know in this manner the absolute number of vibra- 

 tions corresponding to each stroke of the division of the standard 

 diapason. 



