222 OPTICAL PROJECTION 



divisions of a tank, filled respectively with alcohol, water, and 

 heavy brine. Different fluids may also be projected in layers, 

 in a test-tube or small tank. These are experiments which, 

 to avoid any chance of misconception, should be made with 

 the erecting prism, unless shown on a large scale by the shadow 

 method. The level of liquids in differently shaped vessels, is 

 best shown on the large scale : the balance of a small column 

 by a large one can be shown equally well either way, if a prism 

 be at hand. For pressure fountains the shadow method must 

 be used. 



116. Waves and Ripples. The propagation, and cross- 

 ing by reflection from one end of a trough of waves on the 

 surface of a liquid, are easily projected by the shadow method 

 in a trough a few feet long with glass sides. Using the 

 vertical method, and a horizontal tank like fig. 117 with a glass 

 bottom, smaller ripples can be shown by touching the surface 

 with the point of a rod, but the effect is not very satisfactory ; 

 it is better on the surface of methylated spirit than water. 

 More distinct ripples are produced by rapid intermittent con- 

 tact of a pointed wire, which may be produced either by a 

 multiplying wheel actuating an arm, so as to give a rapid in- 

 termittent dip to the bent end of a wire attached to the arm, 

 or by a bent wire attached to the end of a tuning-fork, electri- 

 cally driven. By the latter, sharp ripples are readily produced 

 and focussed, and by adjusting the starting point rather to 

 one side (still better in one of the foci of an elliptical trough), 

 interferences may be shown. 



See also acoustic ripples in Chapter XVII., and by substi- 

 tuting a vessel of mercury for the funnel of the phoneidoscope 

 shown in fig. 142, its surface occupying the position of the 

 soap -film when the funnel is in place, beautiful ripples may 

 be projected by touching the surface with the point of a wire 

 vibrated as just described, 



117. Capillarity. Capillary attraction is projected by dip- 

 ping small tubes of different bores, fixed in a slip of wood, ag 



