On Light- Sensations and the Theory of Forced Vibrations. 491 



position corresponding to Fraunhofer's C-line, the green sensation by a 

 green ribbon near the Fraunhofer 5-line, and similarly the blue and the 

 violet by a ribbon of each colour in the corresponding region of the spectrum 

 In the actual model there are five grey ribbons between every two coloured 

 ones — in the diagram only three are drawn, for clearness. The bar A is 

 made to oscillate by a heavy pendulum B, with a movable bob fixed by a 

 thumb-screw. If this is tuned to the period of the green ribbon, although 

 those on either side respond more or less, the maximum amplitude is attained 

 by the green, which, according to the theory of Thomas Young, is most 

 strongly excited by light of a certain wave-length. 



If the pendulum is tuned for the yellow, then the grey ribbon under 

 neath the yellow portion of the bar indicates by the amplitude of its oscilla- 

 tion the position in the spectrum of the impressed vibration, while the red 

 and green, representing the colour-sensations, swing moderately, being both, 

 as Young said, excited, though to a less degree, by yellow light. It makes 

 the experiment more striking if the bar A is painted with the complete 

 series of spectral colours, and the principal Fraunhofer lines marked upon it 

 in black in their proper positions. 



For some reason, perhaps connected with the fact that the ribbons swing 

 so near each other and that eddies are formed by their edges, friction is 

 relatively greater. at small than at large amplitudes, so that to some extent 

 the effect of its increase on the range of resonance may be seen as the 

 oscillations subside. For greater differences I use a set of oscillators with 

 lighter weights. 



For purposes of demonstration it is necessary to show the effect upon 

 such a system of resonators of light of more than one wave-length. To do 

 this two heavy pendulums, C and D (fig. 2), are connected by light wooden 

 rods, E and F, to the ends of the cross-link G-, from the middle of which a 

 third rod H leads to a crank-pin on the bar A. The heavy bob of the 

 pendulum B being removed, the stem of it, which is quite light, serves as 

 an index to show the compound nature of the motion imparted to the 

 bar A, the movements of which often appear strikingly irregular. But 

 each pendulum is responded to by the resonators in tune with it, as though 

 the others were still. 



Quite apart from any theories of colour-vision, the apparatus demonstrates 

 in a striking manner the phenomena of forced vibrations. The change of 

 phase according as the natural period is greater or less than that of the 

 impressed force is well shown, and it is particularly instructive to start with 

 a considerable difference of period between the pendulums C and D, and 

 gradually bring them into unison. 



VOL. lxxxvi. — B. 2 o 



