124 OPTICAL PROJECTION 



CHAPTER IX 



SCREENS AND OTHER LANTERN ACCESSORIES 



WE liave in the last chapter supposed the lantern in position, 

 and the screen up ; but there are a few matters yet to be con 

 sidered before we can dismiss practical exhibition work with 

 such an assumption. 



67. Screens. There is no dispute at all as to what makes 

 the very best screen, for all kinds of lantern work. It is, a 

 fine smooth surface of white plaster of Paris, and next to that 

 a smoothly whitewashed wall, finished with whiting, and not 

 with lime. Such a surface is both white and opaque, reflect- 

 ing back nearly all the light which falls upon it. Such a 

 surface, as far as I have been able to ascertain from rather 

 rough tests made with polarised light, gives fully 50 per cent, 

 more light than the best white sheet, and 25 per cent more 

 than the very best faced screens that can be made. 



In public institutions, therefore, and especially where 

 physical projections sacrificing much light are made, it is well 

 worth while to reserve a proper space of wall at the back of the 

 platform for such a surface, and to prepare it by a thin coat- 

 ing of fine plaster, carefully whitening thereafter as required. 

 Even for ordinary lantern lectures it would be well worth 

 while, as no one will question who has once seen slides pro- 

 jected upon such a surface 25 feet square. It is a matter of 

 continual surprise to me that, now demonstrations by the 

 lantern have become so customary in all halls and institutions, 

 steps are not taken to provide for their best effect, in a simple 

 and economical manner which nevertheless far surpasses any- 

 thing possible to a casual demonstrator. The white surface 

 can easily be draped over when not required. 



In Germany it has been the practice for some time to pre- 



