12S OPTICAL PROJECTION 



stretched in the making. For physical demonstration more 

 than 10 or 12 feet is not required ; for microscopic work 12 

 to 14 feet is better. Both for the latter, and also for pictures, 

 it is to be noted, that some margin improves the effect ; thus, 

 a 12-feet disc will look much better on a 15-feet screen, 

 than on a 12-feet screen. Eegular lecturers who use sheets, 

 generally prefer to have two ; one 12 to 14 feet, and the other 

 18 to 20 feet square. If only one is used, 15 or 16 feet is the 

 most useful size. 



70. Erecting the Screen. This is the most anxious piece of 

 work of any to a peripatetic lecturer. When his screen is up, 

 he feels at ease. He never knows, in a new place, what con- 

 veniences he may find, or what awkward arrangements he 

 may have to get over. If the place is often used for such 

 exhibitions, his first thought is to look aloft, as he is pretty 

 certain to find a couple of eyes or hooks some distance apart, 

 left by some one or other of his predecessors ; for it is an under- 

 stood courtesy amongst lecturers, that any such hooks or eyes, 

 once placed in awkward positions, are left to smooth the path 

 of successors. If these are found, then, and are at a height 

 and distance which fairly suit the size of sheet he means to 

 use, he has only to find a ladder tall enough, and his task is 

 easy. The eyes must be high enough to raise the bottom of 

 his disc above the heads of the audience, and enough apart to 

 be some inches wider than his sheet how much wider matters 

 little to him. If there are no such attachments, he must put 

 them in himself ; so that it is always wise to make sure first 

 of all, if necessary, that a ladder long enough is available. 

 As to the eyes and hooks, with screwed ends, no exhibitor 

 travels without these. If or when the top pair are all right, 

 the exhibitor's next step is to screw a pair of hooks into the 

 floor, a foot or two wider apart than the bottom corners of his 

 sheet, and just under where these will be. 



There are various ways of fixing up the sheet itself. My 

 experience has been of such an amateur character that it may 



