152 Royal Institution : — 



presses his finger on a string he makes it shorter and tighter, and 

 thus, causing it to vibrate more speedily, augments the pitch. Ima- 

 gine such a player to move, his finger slowly along the string, short- 

 ening it gradually as he draws his bow, the note would rise in pitch 

 by a regular gradation ; there would be no gap intervening between 

 note and note. Here we have the analogue to the continuous spec- 

 trum, whose colours insensibly blend together without gap or in- 

 terruption, from the red of the lowest pitch to the violet of the 

 highest. But suppose the player, instead of gradually shortening 

 his string, to press his finger on a certain point, and to sound 

 the corresponding note ; then to pass on to another point more or 

 less distant, and sound its note; then to another, and so on, thus 

 sounding particular notes separated from each other by gaps which 

 correspond to the intervals of the string passed over ; we should 

 then have the exact analogue of a spectrum composed of separate 

 bright bands with intervals of darkness between them. But this, 

 though a perfectly true and intelligible analogy, is not sufficient for 

 our purpose ; we must look with the mind's eye at the very oscillating 

 atoms of the volatilized metal. Figure these atoms connected by 

 springs of a certain tension, and which, if the atoms are squeezed 

 together, push them asunder, or, if the atoms are drawn apart, pull 

 them together, causing them, before coming to rest, to quiver at a 

 certain definite rate determined by the strength of the spring. Now 

 the volatilized metal which gives us one bright band is to be figured 

 as having its atoms united by springs all of the same tension, its 

 vibrations are all of one kind. The metal which gives us two bands 

 may be figured as having some of its atoms united by springs of one 

 tension, and others by a second series of springs of a different ten- 

 sion. Its vibrations are of two distinct kinds ; so also when we have 

 three or more bands, we are to figure as many distinct sets of springs, 

 each set capable of vibrating in its own particular time and at a dif- 

 ferent rate from the other. If we seize this idea definitely, we shall 

 have no difficulty in dropping the metaphor of springs, and substi- 

 tuting for it mentally the forces by which the atoms act upon each 

 other. Having thus far cleared our way, let us make another effort 

 to advance. 



Here is a pendulum, — a heavy ivory ball suspended from a string. 

 I blow against this ball ; a single puff of my breath moves it a little 

 way from its position of rest ; it swings back towards me, and when 

 it reaches the limit of its swing I puff again. It now swings further ; 

 and thus by timing my puffs I can so accumulate their action as to 

 produce oscillations of large amplitude. The ivory ball here has 

 absorbed the motions which my breath communicated to the air. I 

 now bring the ball to rest. Suppose, instead of my breath, a wave 

 of air to strike against it, and that this wave is followed by a series 

 of others which succeed each other exactly in the same intervals as 

 my puffs ; it is perfectly manifest that these waves would communi- 

 cate their motion to the ball and cause it to swing as the puffs did. 

 And it is equally manifest that this would not be the case if the im- 

 pulses of the waves were not properly timed ; for then the motion 



