16 Dr. A. To pier on the Application of Stroboscopic Disks 

 equations will be 



+ *{(^)-M}.V + x(„ + f) W . 



These axes, however, deduced from several arcs combined, will 

 contain as many unknown quantities as those arcs employed, 

 and must therefore be themselves unknown. It must not be 

 inferred that this method is not to be depended upon. The 

 alternative method, that of Bessel, leads to numerical results and 

 involves no unknown quantities in its results, but is not on 

 that account to be relied upon, as it neglects errors depending 

 upon local attraction which may rise to very great importance. 



J. H. Pratt. 



Calcutta, October 29, 1866. 



III. On the Application of the Principle of Stroboscopic Disks 

 to the Optical Analysis of Vibrating Bodies. By Dr. A. 

 Topler*. 



[With a Plate.] 



SINCE Wheatstone's investigations on the velocity of elec- 

 tricity and on the~chemical harmonicon, the rotating mirror 

 has been frequently used in the optical analysis of the processes 

 which occur in the rapidly successive vibrations of sounding 

 bodies. On the other hand, the vibrograph discovered by Wil- 

 liam Weber, and since then improved in the most varied manner, 

 presents a means suitable for the graphic representation and even 

 measurement of simple and compound vibrations. The advan- 

 tages of both methods for experimental inquiries may be com- 

 bined by the application of a principle which hitherto has only 

 served as the basis of an interesting toy — that is, by the principle 

 of stroboscopic disks. 



Dr. Paris's thaumatrope, Stampfer's stroboscopic disks, and 

 Plateau's anorthoscope are apparatus which, though partly in 

 different ways, produce such optical impressions as depend on 

 the duration of the impression of light in the eye. In regard to 

 the applicability of the stroboscopic principle to optical investi- 



