A THEORY OF COLOUR VISION 



379 



it. Thus by means of a model we shall obtain the results of 

 the mathematical theory without having any recourse to 

 mathematics at all. 



2. AB is a compound pendulum consisting of a long thin 

 brass rod with a knife-edge and a cylindrical weight C. The 

 length of the rod is 148 cm. and its diameter is 6 mm. The 

 weight was made by filling a brass cylinder with lead. It can 

 be moved up and down, and clamped in any position on the 

 lower half of the rod, and thus the period of the pendulum 

 can be altered from i'6o to 2*40 sec. EF is a simple pendulum 

 consisting of a ping-pong ball hung by a thread. This thread 

 is attached to a wire DE, which is fastened to the rod by a 



P-f 



6r 



P«*<xL ir. S.'conJ. 



Fig. 2. 



spring clip at D. The wire DE is parallel to the edge of the 

 knife-edge. 



The length of the simple pendulum was kept constant and 

 its period was 2*03 sec. It is so light that its vibrations do 

 not affect the motion of the compound pendulum appreciably. 



The following experiment was made with this apparatus : 

 the cylindrical weight was clamped high up so that the period 

 of the compound pendulum was 1*63 sec. The compound 

 pendulum was then drawn aside, so that it came against a 

 wooden peg in a position making an angle of about 3 with 

 the vertical, and held until the simple pendulum was perfectly 

 at rest in a vertical position. It was then let go and com- 



