58 OPTICAL PROJECTION 



half empty, the stretcher is lifted off the pin with the toe, 

 and the front end of the board being gently lifted (there is 

 hardly any weight on this end) the legs can be pushed back 

 underneath in the same way, and the front gently lowered on 

 to the ground. 



Skeleton boards are usually made with a base as in figs. 33 

 and 37, the legs or support being hinged to the front of this 

 at B, and also to a point some distance back at A, on the under 

 side of the bottom board. When the front is elevated, this 

 arrangement brings the back of the bottom board forward to 

 c, where it is kept from slipping back by iron pegs slipped 

 into a hole at each side. The base extending back to D, pro- 

 vides security against an overturn ; when half empty the pegs 



w 



FIG. 36 FIG. 37 



are withdrawn by an assistant, and the boards gently lowered 

 and slid back. 



To get a good light (and it is also an element of safety) 

 fair pressure must be used. On a small screen a sufficient 

 light may often be had with 56 Ibs., adding another towards 

 the close. But for a large disc, a pair of bags 36 x 24 x 24 

 will require two \ cwts. to begin, and a third to finish with ; 

 and larger bags say 42x32, for larger orifices, will need 

 three such weights to begin, and a fourth to finish. If gas 

 is no object, even more may be used with gain to the light, 

 but not in proportion. I believe the profitable limit to He, 

 with proper jets, somewhere between 12 and 15 inches 

 pressure. Of course, if jets are used with a lot of so-called 



