188 THE BOOK OF THE LANTERN. 



gelatine than any other medium. Ordinary coloured glass 

 absorbs so much light that it is of very little use in lantern 

 experiments ; and if the operator will try the effect of 

 coloured glass and coloured gelatine side by side he will 

 be surprised at the advantage gained from using the latter. 

 There is one objection to gelatine, and that is, if a very 

 powerful limelight be used it is apt to be affected by the 

 heat ; but this is only the case if the medium in question 

 is kept for a protracted time on the lantern-stage. 



A large number of experiments illustrating the theory 

 of colour and the laws of complimentary tints can be 

 arranged by means of pieces of cardboard with different 

 shaped orifices cut in them, filled in with coloured gelatine. 

 Such examples will easily suggest themselves to any 

 operator with the assistance of a reliable book on the 

 theory of colour. I may mention here a simple arrange- 

 ment for showing the way in which the retina becomes 

 fatigued by looking at an object for some time. 



It consists of a card with 

 two semicircular openings, 

 divided by a horizontal bar 

 (see fig. 49). Over one open- 

 ing, say the lower one, a 

 FIG. 49. piece of card is placed so 



that the image of the upper one alone is projected upon 

 the screen. After looking at this image for some time, 

 the card obscuring the lower opening is suddenly with- 

 drawn, and it is then strange to note how one opening 

 appears to be far duller than the other, although both are 

 in reality equally illuminated. 



