SIX LECTURES ON LIGHT. 



13 



and they are destroyed when the one series is band of light gradually shortening as th 



an odd number of half-wave lengths in ad- tion subsides, until, when the motion ceases, 



vance of the other. With two forks so cir- we hare our luminous disk restored. Weight- 



cumstanced, we obtain those intermittent ing one of the forks as we did before, with a 



shocks of sound separated by pauses of si- 

 lence, to which we give the name of beats. 



I new wish to show you what may be 

 called the optical expression of those beats. 

 Attached to a large tuning-fork, F (Fig. 2), 

 is a small mirror, which shares the vibrations 

 of the fork, and on to the mirror is thrown a 

 thin beam of light, which shares the vibra- 

 tions of the mirror. The beam reflected 

 from the fork is received upon a piece of 

 looking-glass, and thrown back upon the 

 screen, where it stamps itself as a small lu- 

 minous disk. The agitation of the fork by a 



two-cent piece, sometimes the fork; conspire, 

 and then you have the band of light drawn 

 out to its maximum length ; sometimes the? 

 oppose each other, and then you have tha 

 band of light diminished to a circle. Thus, 

 the beats which address the ear express them- 

 selves optically as the alternate elongation and 

 shortening of the band of light. If I move 

 the mirror of this second fork, you have a 

 sinuous line, as before ; but the sinuosities 

 are sometimes deep, and sometimes they al- 

 most disappear, as in Fig. 3, thus expressing 

 the alternate increase and diminution of the 



violin-bow converts that disk into a band of sound, the intensity of which is expressed by 

 light, and if yoi simp:y move your heads to j the depth of the sinuosities. To Lissajous ws 



and fro you cause the image of the band to 

 sweep over the retina, drawing it out to a sin- 

 uous line, thus proving the periodic character 

 of the motion which produces it. By a sweep 

 of the looking-glass, we can also cover the 

 screen from side to side by a luminous scroll, 

 m n, Fig. 2, the depth of the sinuosities indi- 

 cating the amplitude of the vibration. 



Instead of receiving the beam reflected from 

 the fork on a piece of looking-glass, we now 

 receive it upon a second mirror attached t:> a 

 second fork, and cast by it upon the screer. 

 Both forks now act in combination upon the 

 beam. The disk is drawn out, as before, the 



ou e this mode of illustration. 



The pitch of a sound is wholl / ^e 

 by the rapidity of the vibration, ay tLie ,nten- 

 sity'is by the amplitude. The us*: of pitch 

 with the rapidity of the impulses may be illus- 

 trated by the syren, which consists of a per- 

 forated disk rotating over a cylinder into 

 which air is forced, and the end of which is 

 also perforated. When the perforations o 

 the disk coincide with those of the cylinder, a 

 puff escapes ; and, when the puffs succeed 

 each other with sufficient rapidity, the im- 

 | pressions upon the auditory nerve link them- 

 selves together to a continuous musical note. 

 | The more r"apid the rotation of the disk the 

 j quicker is the succession of the impulses, and 

 ' the higher the pitch of the note. Indeed, by 



