THE BOOK OF THE LANTERN. 189 



Another method which illustrates the tiring of the 

 retina, and which also demonstrates the law of compli- 

 mentary colours, can be shown thus : 



A card having a round opening in the centre, filled in 

 with red gelatine, is placed on the lantern stage, and its 

 image allowed to remain upon the sheet for some little 

 time, the attention of the spectator being concentrated 

 upon it. The gelatine is suddenly removed, when al- 

 though the image of the opening is of course perfectly 

 Avhite, it appears to be green, because the retina is tired 

 by its exposure to the red, and can only for a time appre- 

 ciate the remaining colours of the spectrum, which mingled 

 form green ; of course, any primary colour can be chosen 

 for the experiment, and its complimentary tint will be 

 made manifest. This is but a variation of that advertise- 

 ment which has been so common in our streets for some 

 time, where the onlooker is invited to gaze upon cer- 

 tain colours for so many seconds, when the image of the 

 coloured letters looked at will appear, but in their com- 

 plimentary tint, upon the blank space above. 



For experimental work with the lantern, a special form 

 of instrument should be used. I have lately seen a form 

 which I think found its origin in Germany ; in which the 

 objective is so arranged on a sliding base board, that a 

 clear space of some inches is left between it and the lantern 

 condensers ; while a little table between the two serves to 

 support any object whose shadow it is desirable to throw 

 upon the screen. If we are content with the mineral oil 

 lantern, and with such a lantern a great many experiments 

 can be shown, at any rate in a small room, we can ar- 



