278 OPTICAL PROJECTION 



of the beam of light, which also acts as a very long pointer 

 working without weight or friction. Experiments to illus- 

 trate this will be found under ' Sound ' ( 136, 140, 146), 

 and ' Physiology ' ( 126). It is also illustrated in any experi- 

 ment with the reflecting galvanometer. 



156. Reflected Images. The images produced by multiple 

 reflection between parallel surfaces may be illustrated by 

 focussing a small aperture or narrow slit upon the screen, and 

 interposing a very thick piece of plate -glass in the path of the 

 rays at a considerable obliquity. Eeflection between inclined 

 surfaces is best illustrated by the Lantern Kaleidoscope (see 

 fig. 76, p. 146). The nature of a virtual image of an object, 

 appearing to send out rays as if it were a real object, is well 

 shown by turning the lantern somewhat away from the 

 screen, placing a slide in the ordinary stage, and withdrawing 

 the objective front from the nozzle. Near the front of the 

 nozzle adjust the plane-mirror so as to reflect the rays to the 

 screen, and in their path interpose the loose focussing lens, a 

 long-focus one being most convenient. The slide will be 

 focussed on the screen quite distinctly, as if it were really 

 situated behind the mirror. 



157. Refraction. This is best exhibited by a rectangular 

 tank, partly made of tin, with the two sides and one end made 

 of glass, and covered with a piece of tin in which a few slits 

 are cut in different places. The tank should be two inches 

 between the sides, and about 12 inches square is enough for 

 an ordinary room, but for a large hall 18 inches is better. 

 The sides should either have circles blacked out as shown in 

 fig. 146, or tin sides may be cut out, so as to support the 

 glass. 1 The tank is filled exactly to the centre of the circle 

 with water. To show the course of the rays there are two 

 methods, (a) In the water may be stirred a very little milk 

 or a very little eosin or other fluorescent dye ; and the air 



1 The circle is chiefly to illustrate the usual diagram explaining Snell's 

 law. 



