Hypothesis of Colour Vision with Mechanical Illustrations. 345 



bob D hanging from C. A second cord AFE has a second 

 pendulum hanging from F with heavy bob H equal to D. 

 Each pendulum is adjustable in length so as to represent 

 any incident radiation desired. It may be observed that the 

 virtual lengths of these heavy pendulums are CD and F'H 

 respectively. The set of three responders, supposed present 

 at every part of the retina, are represented by the pendulums 

 Rr, G<7, and V>, with very light bobs consisting of paper 

 cones. These are made of such lengths as to correspond, in 

 relative frequencies, with the three colours red, green, and 

 violet. 



These light-bobbed responders have a logarithmic decre- 

 ment of the order used in plotting the resonance curves of 

 fig. 1. The heavy driving pendulums have negligible 

 dampings and so can represent the sustained amplitudes of 

 incident radiations. 



In order that each responder may be equally influenced 

 by the heavy pendulum these light bobs R, Gr, and V are 

 arranged alon^- a straight line through A. (See " Forced 

 Vibrations/' Phil. Mag. Aug. 1918.) 



If only one incident radiation is to be imitated the cord 

 ACB and pendulum CD are sufficient. In order to imitate 

 a second radiation simultaneously incident, the second cord 

 AFE and pendulum FH are provided. This second 

 pendulum influences the responders by means of a wooden 

 bridge near v, connecting the two cords ACB and AFE. 



In order to record the response of these light pendulums 

 under different conditions of stimulation, it is necessary to 

 photograph them when in action. To insure that all these 

 pendulums shall appear to hang from the same point in the 

 photograph, the lens of the camera must be on the line AC 

 produced, as shown in fig. 2. 



6. Results and their Significance. 



Eighteen photographs are here reproduced (PI. VI.) 

 showing time exposures of the responders vibrating under 

 various conditions of excitation. Photographs 1 to 6 show- 

 tests as to colour mixtures. Photographs 7 to 18 show the 

 responders under the action of a single driver, changed in 

 frequency by a number of small steps. 



In dealing with these effects it will be convenient to 

 designate the pendulum or pendulums by the names of 

 colours : thus a driver of such length as to equal that oi' the 

 red responder will be called a " red" driver, and will imitate 

 the emission of red radiation. Pendulums of other lengths 



