92 SCIOPTICON MANUAL. 



THE PHOTODROME. The photodrome, as shown at the 

 Polytechnic, may be made at a very small expense. To 

 produce this effect we require a rapidly moving disk 

 (having one or more slits cut in it) revolving in the place 

 where the slide is placed, and also a larger one placed 

 at some distance the latter representing a wheel, the 

 spokes of which are painted in black on a sheet of white 

 cardboard. When this is made to revolve rapidly in the 

 rays coming from the lantern, all trace of the spokes 

 will be completely lost; but on causing the small disk 

 to revolve at nearly the same speed as the larger, the 

 latter will appear to be moving slowly, although moving 

 rapidly, and by increasing the speed of the smaller 

 wheel, the larger will gradually appear to slacken in 

 speed until it appears to be motionless, and then appa- 

 rently begin to move in an opposite direction to which 

 it is really revolving. 



SILHOUETTES, &c. Paper patterns, silhouettes, &c., 

 suspended by a thread attached to the feet, and twirled 

 before the condenser, give a very amusing and curious 

 effect. 



GALVANIC ACTION. Fill the tank with a solution of 

 nitrate of silver, and introduce at each end two wires 

 from a small battery; from one of the wires a beautiful 

 silver tree will immediately begin to grow. The experi- 

 ment may be varied by substituting acetate of lead for 

 a lead tree. 



Litmus solution, neutralized, will gradually redden 

 around one point, while around the other it will assume 

 a blue tint. 



With a solution of cochineal, the red color will be 

 changed by the acid to a brilliant yellow, and by the 

 ammonia to a rich purple. 



