PROJECTIONS FOR THE SCHOOL-ROOM. 3 



as the outer circle (namely, nine inches) , cut a rabbet 

 upon one side of it so that it will nicely fit into the hole 

 of the larger board, as indicated at c c. 



Make the worked edges, and touching surfaces, quite 

 smooth ; but the outer edge should be made a trifle 

 smaller than the hole, in order to allow the disk to 

 turn freely round in it ; then the hole may be cut in the 

 disk to receive the lens, four or five inches in diameter, 

 whichever it may chance to be. 



Procure a nice piece of thin looking-glass, twelve or 

 fifteen inches long and five inches wide. Fasten it to 

 a back of wood made a little larger than itself, with 

 broad-headed tacks, or bits of wire driven in and the 

 top bent at right angles. This back will need to be 

 an inch thick at the bottom, but may taper like a 

 shingle to the top, where it need not be half an inch 

 thick ; m is the mirror and h is the back in the figure 

 adjoining. 



A common desk hinge h may be used to attach this 

 mirror-mounting to the part c in the figure below. It 

 must be so fastened that the mirror may swing through 

 ninety degrees from a horizontal plane. The accom- 

 panying figure will be sufficiently definite to enable 

 any one to make 

 the whole instru- 

 ment. When the 

 mirror is securely 

 fastened to the 

 part c, the whole 

 can be inserted in 

 the board b b and 

 buttoned in, as is 

 shown at b and b ; 

 these buttons must 



