144 OPTICAL PROJECTION 



vessel, and a flash or two of lightning being still continued. 

 Finally these effects are changed for one showing the moon 

 emerging from behind a cloud. This must be done very 

 gradually, and while the moon-slide is thus worked the scene 

 is gradually dissolved into a bright moonlight scene, with the 

 light effect upon the water. After a few moments a ' snow- 

 slide ' is put in, and when this has fallen for a few seconds 

 the landscape is again dissolved into a winter scene, with 

 snow upon the landscape and ice upon the pond, on which 

 skaters execute their gliding movements ; or this last may be 

 done when the scene is again changed to a night and bonfire 

 effect, with lights in the mill windows. 



Such is an example of what may be called high-class 

 dioramic lantern exhibition ; and if the different slides match 

 and register exactly, the effect is indescribably beautiful. All 

 depends upon that, and also upon having an adequate staff of 

 assistants, for one alone could not possibly conduct all the 

 various operations described. Two would often be required 

 at the lantern itself, irrespective of any acoustic effects ; and 

 in some cases the work would be better done with four 

 lanterns than even three. At the Polytechnic as many as six 

 were occasionally employed, but four would probably perform 

 all that was necessary even in such imposing spectacles as the 

 siege of Delhi, which awakened so much admiration at the 

 time. 



78. Experimental Slides. Besides scenic slides and effects, 

 there are some often shown which are of a semi-experimental 

 character. If two pieces of perforated zinc, or of wire gauze, 

 are mounted in a double-rack chromatrope frame, they pro- 

 duce very interesting effects. Another slide known as the 

 Kaleidotrope, consists of a piece of thin metal pierced with 

 small holes, and hung by a pivot at the centre to the end of 

 a coiled spiral watch-spring. When this is twitched by the 

 forefinger, the metal both spins round and vibrates on the 

 spring, and the effect is many apparently moving circles oi* 



