APPLICATIONS OF THE ELECTRIC LIGHT, 291 



changed, and it was not until many attempts had been made 

 by Duboscq that a real rainbow was obtained, in the follow- 

 ing manner : 



" The electric apparatus supplying the arc," says Saint-Edme, 

 "is placed on a stand of suitable height at 5 metres distance 

 from the curtain, and perpendicularly to the canvas representing 

 the sky. The whole optical apparatus is arranged and fixed in 

 a blackened case, which diffuses no light externally. The first 

 lenses give a beam, which afterwards encounters a screen cut 

 out in the form of a bow. This beam is received by a double 

 convex lens of very short focus, which serves the two purposes 

 of increasing the curvature of the image, and of spreading it out. 

 On leaving this last lens the rays traverse the prism, by which 

 they are dispersed and made to produce the rainbow. The 

 position of the prism is not a matter of indifference ; its angle 

 must be at the top of the incident beam, or otherwise the colours 

 will not be displayed in the order in which they appeared in the 

 rainbow. By this method the rainbow appears luminous even 

 when the stage is fully lighted. 



" It is no difficult matter to imitate peals of thunder in the 

 theatre ; the shops supply tam-tams and elastic sheet-iron for this 

 purpose ; but it is not so easy to make lightnings flash on the 

 stage with anything like a natural effect. At first the pheno- 

 mena was imitated by lighting a narrow zigzag cleft in the scene, 

 with red fire from behind. With the progress of scenic art, ii 

 was necessary to do something better, and by the aid of science 

 the source of light was found in the voltaic arc, which is iden- 

 tical in origin with the lighting itself. But what further had to 

 be found, was some optical arrangement by which the luminous 

 beam could be emitted and cut off at rapid intervals, while 

 giving the zigzag movement characteristic of lighting. For this 

 purpose Duboscq had recourse to a kind of magic mirror, in 

 front of which the electric light was placed. This mirror was 

 concave, and the luminous arc was situated at its locus. The 

 upper carbon was fixed, but the lower carbon could at any re- 

 quired moment be drawn back, when the light would flash out. 

 This could also be done by electro-magnetic attraction, ana as 

 the mirror was held in the hand it was possible,- by shaking it 

 and using the commutator, to obtain currents in various direc 



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