306 SUMMARY OF CURRENT RESEARCHES RELATING TQ 



peculiar method of observation, proposed in its essential features by 

 tbe French physicist Lissajous, I have found it possible to observe the 

 vibrational form of individual points in a violin string, and from this 

 observed form, which is comparatively very simple, to calculate the 

 whole motion of the string, and the intensity of the upper partial ones. 



Look through a hand magnifying glass consisting of a strong 

 convex lens, at any small bright object, as a grain of starch reflecting 

 a flame, and appearing as a fine point of light. Move the lens about 

 while the point of light remains at rest, and the point itself will 

 appear to move. In the apparatus I have employed, which is shown 

 in fig. 47, this lens is fastened to the end of one prong of the tuning- 

 fork G. It is in fact a combination of two achromatic lenses, like 

 those used for the object-glasses of Microscopes. These two lenses 

 may be used alone as a doublet, or be combined with others. When 

 more magnifying power is required, we can introduce behind the 

 metal plate which carries the fork, the tube and eye-piece of a 

 Microscope M of which the doublet then forms the object-glass. 

 This instrument may be called a Vibration Microscope. 



The end of the other prong of the fork is thickened to counter- , 

 balance the weight of the doublet. The iron loop B, which is clamped 

 on to one prong, serves to alter the pitch of the fork slightly ; we flatten 

 the pitch by moving the loop towards the end of the prong. E is an 

 electro-magnet by which the fork is kept in constant uniform vibra- 

 tion on passing intermittent electrical currents through its wire coils. 



When the instrument is so arranged that a fixed luminous point 

 may be clearly seen through it, and the fork is set in vibration, the 

 doublet moves periodically up and down in pendular vibrations. 

 The observer, however, appears to see the luminous point itself 

 vibrate, and, since the separate vibrations succeed each other so 

 rapidly that the impression on the eye cannot die away during the 

 time of a whole vibration, the path of the luminous point appears as a 

 fixed straight line, increasing in length with the excursions of the fork. 



The grain of starch which reflects the light to be seen, is then 

 fastened to the resonant body whose vibrations we intend to observe, 

 in such a way that the grain moves backwards and forwards hori- 

 zontally, while the doublet moves up and down vertically. When 

 both motions take place at once, the observer sees the real horizontal 

 motion of the luminous point combined with its apparent vertical 

 motion, and the combination results in an apparent curvilinear motion. 

 The field of vision in the Microscope then shows an apparently steady 

 and unchangeable bright curve, when either the periodic times of the 

 vibrations of the grain of starch and of the tuning-fork are exactly 

 equal, or one is exactly two or three or four times as great as the 

 other, because in this case the luminous point passes over exactly the 

 same path every one or every two, three, or four vibrations. If these 

 ratios of the vibrational numbers are not exactly perfect, the curves 

 alter slowly, and the effect to the eye is as if they were drawn on the 

 surface of a transparent cylinder which slowly revolved on its axis. 

 This slow displacement of the apparent curves is not disadvantageous, 

 as it allows the observer to see them in different positions. But if 



