APPARATUS FOE TIMING LIGHT. 38 



of light. As the light enters the tube, the rays, of course, 

 are diverging. Near the entrance they pass through a 

 lens by which they are made parallel. A little farther on 

 they pass through another lens, by which, from, being par- 

 allel, they are made to converge, but before coming to a 

 focus they strike the plate of glass, M. This glass, though 

 not strictly a mirror not being silvered on the back re- 

 flects a large portion of the rays, turning them into the main 

 tube, but without, however, changing their convergency. 



On the other side of the tube is a system of clock-work 

 moved by a weight attached to a cord that is wound round 

 a drum seen at the end. Those who are interested in 

 tracing out the connections of machinery will see that 

 there are four axles in this work. The first is the axle of 

 the drum. Near the end of this axle, toward the right, is 

 a large toothed icheel, which carries a small wheel upon tho 

 end of the next axle. On the left-hand end of this second 

 axle is another large wheel, which carries a small wheel on 

 the third axle. This same system of large wheels carrying 

 small ones is carried through the fourth and fifth axles, 

 and thus a very great velocity is imparted to the fifth by 

 the descent of the weight. 



On the left-hand end of the fifth axle is a large wheel, 

 which you see enters into the tube through a slit made in 

 the side of the tube for admitting it. The margin, or cir- 

 cumference of this wheel, is cut into alternate notches and 

 teeth, square in form and equal to each other, and the wheel 

 is so adjusted in respect to the tube at the point where the 

 beam of light passing through is just coming to a focus, 

 that each tooth, as it moves by, shall stop a beam of light, 

 while the notch that follows shall allow it to pass. Thus, 

 when the wheel is rapidly revolving, a succession of flashes 

 will pass out through the tube, following each other with 

 inconceivable rapidity. 



R ? 



