126 OPTICAL PROJECTION 



from creases or buckles. The seam should be judiciously 

 placed. It is best to run horizontally, but in any case should 

 not cross the middle. For a 15-feet screen a seam 10 feet 

 from the top would be best ; a 20-feet sheet would be best made 

 of one piece 10 feet wide, and 5 feet top and bottom. The eye 

 naturally goes to the centre of the picture, and in this way 

 the seam is less conspicuous. 



The brightest portable screens are those made of some 

 flexible material and faced with white paper, the latter being 

 still better faced with some opaque material. For this latter 

 improvement demonstrators are indebted to Mr. JohiiD. Mason. 

 This gentleman himself an experienced lanternist was kind 

 enough to take a great deal of pains in obtaining various 

 samples of white paper for me, with a view to a screen for 

 microscopic projections. The best of these were a perceptible 

 improvement over any faced screen I had been able to obtain 

 before, but neither of us were satisfied, and I suggested trying 

 distemper on the paper. I tried various preparations, and Mr. 

 Mason others, but with the common result, that anything 

 really white speedily flaked off the surface. At length it 

 occurred to Mr. Mason to apply to the paper-hangers for 

 samples of their mineral-faced papers ; and some of the best 

 of these I found to give a brilliance considerably beyond any 

 surface of paper alone, while the expense is only about 50 per 

 cent, more than of any ordinary paper-faced screen. It is not 

 equal to a wall surface, but comes the nearest to it of any 

 portable screen I am acquainted with. These screens have 

 since become well known to, and highly approved of by all 

 the best London opticians. The largest I am aware of was 

 20 feet square, and it is much to be wished that halls which 

 cannot or will not provide a wall surface, should provide a 

 screen of this kind, to be rolled up when not in use. For an 

 expense of a very few pounds once for all, lecturers would then 

 be spared the most anxious and troublesome part of their pre- 

 parations, and the effect of their pictures would be much 



