HISTORICAL REVIEW OF MODERN THEORIES 199 



an abrupt change of phase of half a period (Fig. 56). Hence if a 

 disturbance of this character were to act on a system of which the 

 natural period corresponds to that of the yellow light there would be 

 little response, because the disturbance produced during the interval 

 between two consecutive minima would be neutralised by the disturbance 

 produced during the next interval. An illustration will make the point 

 clear. If a succession of correctly timed small impulses be given to a 

 heavy pendulum the pendulum will in time be set in violent motion. 

 If, however, after giving, e.g. four of these timed impulses the timing 

 is suddenly altered by half a period the new impulses will tend to check 

 the motion already communicated, and after four impulses the pendulum 

 will be brought to rest 1 . 



Fig. 56. Combination of two sine curves. 



In 1801, in the Bakerian Lecture before the Royal Society, Thomas 

 Young 2 propounded the theory which now commonly bears the name 

 of the Young-Helmholtz Theory. His third hypothesis of " the theory 

 of light and colours " was that the sensations of different colours depend 

 upon the different frequencies of the vibrations of the light which falls 

 upon the retina. Since it is well nigh inconceivable that each sensitive 

 point of the retina possesses innumerable parts capable of responding 

 to the individual vibrations, he concluded from Newton's researches 

 that three parts would suffice to account for all the colour sensations, 

 including white. He chose three chief colours, red, yellow, and blue, 

 the wave-lengths of which were in the ratios 8:7:6. Each affected a 

 corresponding part of the sensitive mechanism. In 1802, after Wollas- 

 ton's description of the spectrum, he chose red, green, and violet, which 

 he confirmed on experimental grounds in 1807. He considered that 

 the simplest conception of the fundamental " parts " was three kinds 

 of nerve fibre. Excitation of the first aroused the sensation of red, 

 of the second green, and of the third violet. The first type of fibre was 

 most excited by lights of long wave-length, the third by those of short 

 wave-length, and the second by those of intermediate wave-length, but 



1 I am indebted to Prof. W. Watson, F.&.S., for this explanation. 



2 Sec Roy. Land. Ophth. Hosp. Rep. xix. i. 1, 1913. 



