34 ILLUMINATION. 



about six inches wide and twelve long, A, fig. 17, is 

 fitted with levelling screws,, + + + +, at its four 

 corners. On it a small French clock, M, is placed, a 

 boxwood wheel one inch in circumference having 

 been previously attached to the hour hand axle. 



A sheet of white paper, with a line NS drawn 

 across it, is gummed to the back of a square of thick 

 plate glass, D, the whole being fastened to A, as in 

 the figure. 



A ball and socket joint is fixed at E, the ball 

 having a hole half way through it, the orifice being 

 on a level with the top of the glass plate, and in a 

 direct line with NS. 



A wooden pillar F, the top being cut obliquely is 

 placed as shown; on it is fastened a thick brass 

 plate, by a binding screw, B, passing through the 

 slot at one extremity, the other extremity being 

 perforated to receive a knitting needle, H. 



This knitting needle must be perfectly straight, 

 and work very smoothly in E and G. On it between 

 E and G a boxwood wheel, I, four inches in circum- 

 ference is fixed, and above G a small mirror K. 



A motion four times as slow as that of the hour 

 hand of the clock is imparted to the needle and thus 

 to the mirror, by a silk cord passing round the box- 

 wood wheels M and I. 



An elastic band and a small hook is the best 

 means of keeping the cord tight, it should always be 

 detached when the instrument is not in use. 



Another and large mirror, L, mounted to allow of 

 its rotation in all directions, and elevation to any 



