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



commencement of the experiment the disk is indeed somewhat 

 behind, it then gets in advance, and sinks gradually to the nor- 

 mal velocity — which is easily explained. The observer can make 

 the observations without any hindrance. If the apparent vibra- 

 tions follow too rapidly, a slight correction of the pendulum or 

 the metronome is sufficient. The vibrations succeeded so rapidly 

 that there was no occasion for seeking another mode of regula- 

 ting the clockwork. Unfortunately there must be a fresh regu- 

 lation for each new object with a different tone. 



The observations are liable to errors, which fortunately are 

 avoidable. Looking with the naked eye through the row of 

 holes, which seems like a transparent circle, the images of the 

 vibrating objects appear wanting in definition if the holes have 

 a diameter which is considerable as compared with the distance 

 between two adjacent holes ; for the flash of the image during the 

 passage of an aperture in front of the pupil is not instantaneous. 

 If the ratio of the diameter of a hole to its distance from the 

 next is 1 : p, and if the disk is rightly tuned for the sounding 



body, during the passage of a hole - vibration of an object 



elapses. The image must be the more distorted, and run into 

 the succeeding phase, the smaller p is. I was surprised at ob- 

 taining perfectly distinct images of a great number of vibrating 

 bodies, even when p did not exceed from 10 to 12. On a little 

 consideration this is explicable in the following manner : — Assu- 

 ming that the rotating holes are circular and exactly of the same 

 diameter as the pupil, the intensity of the impression of light 

 during the passage of a hole is not constant ; for, while the hole 

 in question gradually slides in front of the pupil, the intensity 

 rapidly increases from to a definite maximum value, and the 

 intensity of light will always depend on the area of the overlap-" 

 ping part of both circles (hole and pupil). The maximum of 

 intensity will in fact only last for a moment, and then rapidly 

 decrease. The impression of light remaining on the eye must 

 obviously correspond to that position of the vibrating object 

 which coincides with the maximum brightness. The adjacent 

 positions of the object, which during the passage of a hole are 

 still visible, will recede, for they correspond to a smaller bright- 

 ness. Hence it is that with holes of not more than the diameter 

 of the pupil, even very fine objects, such as vibrating strings, can 

 be very well observed whenp does not amount to more than from 

 20 to 25. From this may also be explained what at first sight 

 seems paradoxical, namely the fact that holes of double the dia- 

 meter of the pupil give indistinct pictures if their mutual dis- 

 tance is increased to double, or even more. For observations 

 with the naked eye, holes of 3 to 4 millims. diameter are best 



