18 Dr. Topler on the Application of the Principle of Stroboscopic 



of the sounding body can be optically reproduced, made as much 

 slower as we like, and either in a negative or positive direction. 

 The eye will in each case observe just as many apparent vibra- 

 tions as the ear would experience impulses if the disk were 

 sounded as a siren so that interference would take place be- 

 tween the two tones produced. It must be expressly remarked 

 that this conclusion is only valid for the case in which the ob- 

 server's eye is close behind the rotating disk. 



The principle is so simple and universally known that it might 

 be supposed to have been more frequently used for studying 

 phenomena of vibration. In fact, when freed from some optical 

 sources of error to be subsequently mentioned, it furnishes a very 

 suitable means of constructing a universal vibroscope which can 

 indicate the difference of phase both of space and of time. The 

 advantages over the rotating mirror and the vibrograph are 

 obvious; for the former only furnishes drawn-out successive 

 pictures — while the vibrograph only gives the form of the curve 

 of vibration of a single vibrating point, but never allows an 

 investigation of the whole body as regards changes in form, op- 

 tical properties, &c. As I have not met with any application 

 of this principle in my reading in experimental optics, I will 

 attempt in the following communication to indicate the value of 

 the method for analytical purposes. 



I made my first experiments with tuning-forks. A tuning- 

 fork which gave the next lower octave of the sounding object 

 had on one limb a thin metal disk with a fine aperture, the other 

 limb being balanced by a small weight. With a brisk excitation 

 of the tuning-fork, the eye, placed directly behind the position 

 of equilibrium of the vibrating aperture, sees, indeed, the pheno- 

 mena first mentioned, if a second very small aperture is brought 

 in front of the eye. Experiments made in this way are, however, 

 very inconvenient, because the intensity of light is so materially 

 diminished, and, on the other hand, because a continuous strong 

 excitation of the tuning-fork cannot be obtained without compli- 

 cated arrangements. Hence I turned to rotating disks with 

 concentric series of holes, as used in every form of siren. The 

 greatest difficulty here consists doubtless in imparting to the disk 

 any desired velocity, and retaining it constant during the entire 

 duration of the observation. This might easily be effected by 

 means of clockwork, — perhaps best of all by a small magneto- 

 electrical machine, which in turn might be regulated by a rheo- 

 stat. But leaving to practice a more perfect arrangement of such 

 a mechanism, I limit myself to describing one which, with a 

 little practice, may be used with perfect success for vibroscopic 

 purposes. 



A, fig. 1, Plate I., represents a small clockwork, as used for 



