SCREENS AND LANTERN ACCESSORIES 125 



pare portable plaster of Paris screens in an iron frame, of about 

 5 feet diameter, for microscopic projections ; and in this way 

 results have been obtained, which otherwise the instruments 

 employed would have been incapable of producing. 



The next best are portable screens of flexible material 

 faced with some similar opaque coating. For any very special 

 occasion indeed, the best plan is to have a sheet made of the 

 required size it is never worth while taking the trouble 

 unless the screen is wanted at least 20 feet square out of the 

 stoutest unbleached calico, and tack it tightly on a light 

 wooden frame put up in the hall, so that it is tense all over. 

 Then the whole should first have a coating of size ; and when 

 this is dry, several coats of the cleanest whiting mixed also 

 with size. Screens thus stretched, and distempered for the 

 occasion, give such a good effect, that it is worth while to take 

 the pains for any public affair of special importance ; and as 

 the sheet itself is portable enough, and will do over and over 

 again, the cost is not great. For a course of lectures in an 

 important place, where the screen can be left up, such a plan 

 will amply repay the trouble. 



I have read in different books that a screen of this kind 

 can be prepared which will roll and unroll upon a roller, and 

 keep clean. My own efforts have failed in making any, after 

 many trials ; but it is possible that some workmen with 

 technical skill in distemper might succeed in such a screen. 

 I know that the old Polytechnic screen was prepared in some 

 such way ; but that always appeared to me a very dirty one, 

 and rather an example to be avoided than otherwise. 



For ordinary lantern work, a simple screen of white linen 

 or thick calico answers all purposes so long as it is kept clean, 

 and it can be washed when necessary. This sort of screen, 

 being the most portable, is used by travelling lecturers more often 

 than any other. Up to nearly 10 feet square, material can be 

 got for a screen in one piece. Above that it must be sewn 

 together, and this must be done very carefully indeed, to keep 



