26 On the Optical Analysis of Vibrating Bodies. 



double image is produced, of always simultaneously viewing 

 those conditions of phase which are just half a vibration from 

 each other, which is very essential for estimating the motion. 



Under other circumstances a partial resolution of the images 

 is obtained, which may sometimes be. interesting. Assuming 

 that the vibrating object executes n vibrations in a second, while 

 m holes simultaneously pass in front of the eye, periodic changes 

 are observed in the optical image, provided n and m almost ex- 

 actly represent a simple ratio of numbers, so that the tones in 

 question form an harmonic interval. The convulsive movements 

 of the flame are of a complicated kind, and must be specially in- 

 vestigated in each case. Without entering further into the 

 subject, it will be seen that the phenomena reproduce in an 

 optical sense what in acoustics are called combination tones. 



(7) By the following experiment the application of the vibro- 

 scope acquires an additional extension. In one and the same 

 annular zone of the disk B, fig. 1, two different divisions of holes 

 may clearly be simultaneously introduced, so that they may si- 

 multaneously come into operation in rotation. In this way two 

 objects, differing in their duration of vibration, may be simulta- 

 neously analyzed; for the different divisions of holes, strangely 

 enough, scarcely interfere with each other in their optical action. 



(8) The above-described experiments are vastly improved if 

 an intermittent illumination is established through the rotating 

 disk, and on the same principles as those developed for direct 

 vision. 



Let H, fig. 4, denote the mirror of an ordinary heliostat, as used 

 for a solar microscope. The lens L gives at O a small solar image, 

 the rays of which diverge into the darkened place of observation. 

 Let the vibroscope, of which in fig. 4 only the disk B with its 

 axis a is drawn, be so placed that the cone of solar rays can just 

 pass through one of the series of holes. All side light is cut off 

 by having on the tube of the solar microscope a piece of tubing 

 which almost reaches to the disk B. Setting this in rotation, 

 the issuing cone of rays appears constant when a certain velocity 

 has been attained. If now with this light any sounding body, 

 for instance a string S, be observed which is almost in unison 

 with the disk, the vibrations may be studied with a distinctness 

 and degree of facility which leaves nothing to be desired. 

 Not only have we the advantage that many persons can simulta- 

 neously observe the object, but even microscopic objects can be 

 observed with the lens at any degree of closeness. With strings 

 and tuning-forks the previously mentioned duplication of the 

 images can also be shown in the shade figures enlarged 

 on a screen. The form of the apparently slowly vibrating 

 objects appears much more distinct on the screen than might 



