250 



OPTICAL PROJECTION 



tained liquid. These may be projected in various ways. 

 If a thin and pretty shallow bell-glass can be found, about 5 

 inches diameter, on a short leg or foot, the latter may be 

 cemented to the centre of a massive plate of glass, and placed 

 on the vertical attachment. The field lens should be removed 

 from the latter, as the bell when filled with liquid will form 

 its own converging lens. The bell being filled with the liquid 

 selected water, alcohol, ether, or carbon di-sulphide the 

 surface is focussed by a scrap of paper thrown on it and after- 

 wards removed, and the bell excited by a violin bow, or in 



many cases the finger 

 rubbed with resin 

 will suffice. In this 

 case the fluid must 

 be clear. 



But on the whole 

 reflected images are 

 preferable. Fig. 132 

 shows a very simple 

 arrangement, pro- 

 vided a ceiling, or 

 any kind of overhead 

 screen can be utilised 



for the image. Slightly diverging rays are reflected downwards 

 from the nozzle of the lantern or optical front N, by the plane 

 mirror A, arranged so as not to scatter the light beyond the 

 surface of the liquid in the goblet or glass c, and the reflected 

 rays are focussed (for the surface of the liquid) upon the ceiling 

 or screen D. Instead of rays from the nozzle N, the parallel 

 or slightly convergent beam from condensers may be employed. 

 In this case the best effect is produced by employing water 

 blackened with writing ink, so that rays reflected from the 

 surface alone may be utilised. A violin bow is neither neces- 

 sary nor desirable, being far too energetic ; the resined fore- 

 finger dipped in the water is ample, and the nodes will follow 



FIG. 132 



