174 OPTICAL PROJECTION 



whose diverging rays large shadow projections will be well 

 denned, as described in Chapter XIV. 



Such an attachment can either be fitted into the jacket c D 

 of the optical front (fig. 94), or if the back lens be made of 

 longer focus, it may simply be fitted into the small end nozzle 

 N of that piece of apparatus. 



89. Optical Accessories. For general focussing and other 

 purposes there should be provided, wherever physical demon- 

 stration is attempted, at least two rod or pillar stands, made 

 by screwing a half-inch brass tube into a heavy foot, covered 



FIG. 96 FIG. 97 



at the bottom with baize. On these should slide tightly 

 sockets as shown in fig. 96. Small holes, A A, are bored trans- 

 versely through the tube of each socket, and saw-cuts made 

 from the ends of the tube down to them ; then a slight pinch 

 in a pair of gas-pliers will tighten the fit at any time. To 

 one side is soldered the smaller tube B c. All these are one 

 gauge, so that every piece of apparatus will fit into any 

 socket-tube B c, and turn round in it to any position. One or 

 two sockets should be provided modified as in fig. 97, a perpen- 

 dicular socket A B being soldered at the end of the projecting 

 spur- tube. The gauge of the aperture in A B is to be the same as 

 in B c of fig. 96. Thus any piece of apparatus may be placed 

 therein and be capable of turning on a perpendicular axis. 

 Into sockets thus arranged we can fit any apparatus, by 



