LIGHT. 95 



of light from a lantern or from the sun may.be directed 

 while the water flows. For the success of this experi- 

 ment it is necessary that the orifice should be round, 

 smooth, and thin, and the body of water in the tank 

 must not be disturbed by currents. In the figure, 

 water is admitted at F, while at G there is a partition 

 with a good many orifices in it through which the water 

 flows, keeping it at a constant height I. When, there- 

 fore, the light is concentrated upon the orifice H, it is 

 not scattered, but lights up the whole of the curved 

 stream, giving it the appearance of molten silver. If 

 colored glasses are interposed back of E, D, the color 

 of the stream will also correspondingly change, with 

 very pleasing effects. 



Fig. 71 represents still another form of this experi- 

 ment, in which the vertical attachment to the lantern 

 is used. A vertical fountain jet is opened in the 

 ascending beam from the lantern. The falling water 

 is beautifully illuminated. Plates of colored glass 

 may be used, as before. 



MIRAGE. 



Direct the beam from the porte lumUre, so that it is 

 horizontal or nearly so. Put in a diaphragm with a 

 hole about half an inch in diameter, or less, as the 

 first condition is to have a small beam of parallel rays. 

 No lens will be needed. Next heat a brick, or, still 

 better, a poker or any convenient piece of metal that 

 is a foot long or more, until it is nearly to a red heat ; 

 then place it just in front of the diaphragm and parallel 

 with the beam and about a quarter of an inch below 

 it or to one side of it. The current of heated air will 

 so deflect some of the light as to very much elongate 

 the bright spot upon the screen, or even present 

 another one some inches distant from the first. 



