i^o OPTICAL PROJECTION 



have a hook or two along the bottom of the sheet, which, if 

 necessary, can be laced to another piece of line stretched be- 

 tween the hooks in the floor : but this last is seldom done. 



It is more usual to sew rings on the sides instead of hooks ; 

 but in t*hat case the lacing twine has to be passed through 

 every ring ; whilst with open hooks it can be ' caught ' through 

 them in an instant, and the lacing only takes a third of the 

 time. Some exhibitors prefer the line at the top edge of the 

 sheet to be sewn or hemmed to it ; but I prefer the loose line 

 (1) because the stretch of the line and sheet can always be 

 precisely adjusted this way, and is sure to be evenly distributed 

 all along the line ; and (2) because if the line has to be re- 

 newed, there is less trouble. Some again never use pulleys, 

 but pass their lines direct through the eyes aloft. This, how- 

 ever, greatly hinders proper stretching, and increases the force 

 necessary to be used, besides wearing out the lines by friction. 

 Moreover, a pulley can be fitted with studs to fit on a pin at 

 the end of a long pole, so as to be hooked on to its eye aloft 

 even if no ladder is available. 



Small pulley blocks suitable for these purposes are easiest 

 obtainable at the class of shops often called ' model dockyards,' 

 where parts of naval models are sold. 



Occasionally it may happen that the eyes, or only available 

 places for them, are at the sides of a wide hall, many feet 

 away from the edge of the largest sheet that can be used, and 

 at such a distance that the top edge would sag down a great 

 deal, with the tightest stretch that can safely be put upon the 

 lines and pulleys. The remedy is very simple. After getting 

 the sheet as high as is safe, two slender wooden strips are 

 procured and placed as struts or props under the line, about 

 a foot outside the sheet on each side. In such a case the sides 

 will be laced to these props. 



Faced screens are furnished with three rings on the rod 

 at the top, and if necessary can be got up in the same way, 

 drawing up a line run through these rings. Of course no 



