4 THE ART OF PROJECTING. 



not bind upon the part c, as this must have an easy 

 rotation in its place, though they need to be tight in 

 the board b ; three of them will be enough. Again, 

 a string must be attached to the end of m, passed 

 through a small hole in c, and tied to a tight-fitting 

 thumb peg at d. As the peg is turned the mirror will 

 be raised or lowered. A short lever v must be made 

 fast to some part of c with which to turn the whole fix- 

 ture around as the sun moves. The ray of light / can 

 then be always kept where it is wanted. 



If the window-sill be no more than two or three feet 

 from the floor, it will be better to have this fixture 

 either put into a window shutter, or to remove a pane 

 of glass at the proper place and fasten the board b b 

 into it. In this case it will be necessary to have a cap 

 to place over the hole when it is not in use. 



The lenses will need to be purchased; and for a 

 beginning I recommend a cosmorama lens five or six 

 inches in diameter and with a focal length of eighteen 

 or twenty inches j a plano-convex lens of two and a 

 half or three inches diameter and eight or nine inch 

 focus ; also a pocket botanical glass with focus of one 

 or two inches. These three lenses should cost no 

 more than six dollars, if the two former are unmounted. 

 If one has got a magic lantern, or a sciopticon, the 

 lenses in that will answer admirably. Take one of the 

 glasses of the compound condenser and fasten it into 

 the orifice of the porte lumiere with its convex side out ; 

 then, taking out the front lenses, hold them with one 

 hand in the path of the divergent beam of light from 

 the porte lumiere, and distant but four or five inches 

 from it, and with the other hand hold some object 

 between it and the larger lens ; by moving the lens or 

 the object a little, a sharp outline of it will be observed 



