4 THE ART OF PROJECTING. 



not bind upon the part ^, 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 <r, and tied to a tight-fitting 

 th\imb 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 

 ^hen be always kept where it is wanted. 



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

 irom 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 ; 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 ihe porie lutfiiere with its convex side out; 



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



hand in the path ot 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 



