Dynamical Theory of Heat and Light. 



23 



through a large lecture-hall, is had by taking a thin wooden 

 board, cut to any chosen shape, with the corner edges of the 

 boundary smoothly rounded, and winding a stout black cord 

 round and round it many times, beginning with one end fixed 

 to any point, I, of the board. If the pressure of the cord on 

 the edges were perfectly Motionless, the cord would, at every 

 turn round the border, place itself so as to fulfil the law of 

 equal angles of incidence and reflection, modified in virtue 

 of the thickness of the board. For stability, it would be 

 necessary to fix points of the cord to the board by staples 

 pushed in over it at sufficiently frequent intervals, care being- 

 taken that at no point is the cord disturbed from its proper 

 straight line by the staple. [Boards of a considerable variety 



Fior. 4. 



of shape with cords thus wound on them were shown as 

 illustrations of the lecture.] 



§ 36. A very easy way of drawing accurately the path of a. 

 particle moving in a plane and reflected from a bounding 

 wall of any shape, provided only that it is not concave 

 externally in any part, is furnished by a somewhat interesting 

 kinematical method illustrated by the accompanying diagram 

 (fig. 4). It is easily realized by using two equal and similar 

 pieces of board, cut to any desired figure, one of them being- 

 turned upside down relatively to the other, so that when the 

 two are placed together with corresponding points in contact, 

 each is the image of the other relative to the plane of contact 

 regarded as a mirror. Sufficiently close corresponding points 

 should be accurately marked on the boundaries of the two 

 figures, and this allows great accuracy to be obtained in the 

 drawing of the free path after each reflection. The diagram 



