on Visible Vibration. 



197 



82. When a soda-water glass was partially filled with 

 mercury until it produced, by friction with the moist finger 

 round the edge, the note E flat ; on looking down upon the 

 surface the figure 2 presented itself. 



Fi<?. 2. 



83. The surface is broken up into concentric bands of 

 undulae, while there is at the same time a star of four 

 points revolving in the direction, and with the velocity of 

 the finger. The production of this star seems dependent 

 on the number of nodes in the glass, and, when once pro- 

 duced, is projected in a circular course, by the vibratory 

 changes, in the form of the glass. 



84. The manner in which the undulse seem to embrace 

 each separate radius of the star, conveys an idea of the inter- 

 ference of four series or systems of undulations, proceeding 

 severally from the centres of the four ventral segments 

 towards the centre of the mercurial surface, and then acquir- 

 ing a circular motion, subsequent to their propulsion from 

 the glass, towards the centre. 



85. It is difficult to form a correct judgment by engraved 

 figures ; but it will assist the conception to state, that the 

 divisions between the light and dark portions of each radius 

 are occasioned by a change in the direction of the undulae, 

 so that the phase or inclined side of each little hillock of 

 mercury, presented towards the light, being different to 

 those presented by the adjoining hillocks, the reflexion 

 reaches the eye under different circumstances, and presents 

 the appearance of a brilliant twinkling reflexion on the 

 one hand, and of comparative darkness on the other. In 



