LIGHT AND SOUND. 



would interfere with each other, but light polarised 

 in one plane cannot interfere with that polarised in 

 the plane perpendicular to it. On passing through 

 the analyser, R, the two rays, O and E, are sub- 

 divided into four, as in the figure ; the two, EE, 

 OE, which emerge together, being polarised in the 

 same plane, which is at right angles to the plane of 

 the other pair, EO, OO. As the members of a pair 

 are in different phases, they will interfere with each 

 other, and colours will be produced by one pair 

 which will be the same as would be produced by 

 the other pair if their plane could be turned through 

 90 ; or, in other words, the colours of one pair are 

 complementary to those of the other. If the 

 analyser be made to revolve round the line AB 

 as an axis, all possible gradations of colour are 

 produced. The phenomena of polarised light have 

 proved that the waves of light are not backward 

 and forward waves, like those of air, but transversal 

 that is, up and down, like waves of water, or 

 from side to side. The polarisation of light is the 

 resolution of the vibrations of each particle into 

 two, at right angles to each other, which produce 

 waves travelling in different directions. If one of 

 these can be isolated from the other, the light is 

 said to be polarised. 



SOUND. 



Sound (Latin, sonitus; French, son; German, 

 schalt) is the name given to the sensation produced 

 in the organs of hearing by the vibrations of the 

 air or other elastic medium with which they are 

 in contact. The name is also applied to the 

 physical cause of the sensation ; and that branch 

 of Natural Philosophy which treats of the laws of 

 sound is called Acoustics, from a Greek word 

 meaning to hear. 



That the existence of an elastic medium is 

 necessary for the production of sound, is proved 

 by the fact, that a bell rung in the exhausted 

 receiver of an air-pump cannot be heard. But 

 many other substances besides air can convey 

 sound. Divers below water can hear even a very 

 feeble sound if it is produced under the water. 

 And it is well known that bars of wood and metal 

 transmit sound with great distinctness. If the ear 

 be placed at one extremity of a long piece of wood, 

 it will readily hear a slight tap made at the other 

 end, much too feeble to be conveyed the same dis- 

 tance by the air. 



Air, however, is the medium by which sound 

 is generally conveyed to the ear, and the rate 

 at which sound travels in air can be easily 

 ascertained in the following manner. Let a 

 cannon be fired at a considerable distance from 

 an observer, who counts the number of seconds 

 between the time of his seeing the flash and his 

 hearing the report. The velocity of light is so 

 inconceivably great, that we may, without appre- 

 ciable error, suppose that he sees the flash at the 

 very instant it is produced. The number of 

 seconds is therefore the time that sound takes to 

 travel the distance between the cannon and the 

 observer, and if this distance be measured, the 

 velocity of sound in air is determined. It has 

 thus been found to be in dry air at the freezing- 

 point about 1082-7 feet per second, or rather less 

 than 1 2^ miles a minute; but the rate depends 



slightly on the temperature of the air, being greater 

 as the temperature is higher. Thus, at 59 Fah- 

 renheit the velocity is 1112-2. Again, when we 

 know the rate at which sound travels, and the 

 time it has taken to reach our ear, we can find 

 the distance at which it was produced. For in- 

 stance, if, after seeing a flash of lightning, we count 

 3i seconds before hearing the thunder, we know, 

 if the temperature of the air at the time is 59 F., 

 that the discharge has taken place at a distance 

 of 3^ times 1112-2 feet, or 3892-7 feet. It is evi- 

 dent that it cannot be the same particles of air 

 that are set in motion by the sounding body which 

 strike the ear to produce the sensation of sound, 

 for this would require the air particles to be trans- 

 ported at the rate of 12^ miles a minute, or 750 

 miles an hour, which is six times faster than the 

 rate of the most violent hurricane that ever blows. 

 The motion is a vibratory one, so that the dis- 

 turbed particles move backwards and forwards 

 through a small distance, and similarly disturb 

 the neighbouring particles. This is a wave-motion, 

 in which the displacement of each particle is in 

 the direction in which the wave travels, whereas 

 in waves on the surface of water, and in the 

 ethereal waves which constitute light, the particles 

 move at right angles to the direction in which the 

 waves travel. Sound-waves are therefore waves 

 of condensation and rarefaction. And the dis- 

 tance between one point of greatest condensation 

 and the next, is the wave-length. The range 

 through which each particle moves backwards and 

 forwards is called the amplitude of the vibration, 

 which is quite independent of the wave-length. 

 The extent of this range will manifestly be in- 

 creased, if the exciting cause of the atmospheric 

 disturbance be greater, and the intensity or loud- 

 ness of the sound will be greater. Thus, if all 

 other conditions remain the same, the loudness of 

 sound is measured by the amplitude of the vibra- 

 tion, and is proportional to the square of this 

 amplitude, so that a double amplitude gives a four- 

 fold intensity. When a series of waves is pro- 

 pagated from any point, the amplitude diminishes 

 in the same proportion as the distance increases, 

 and thus the intensity of sound diminishes as the 

 square of the distance increases. 



The air-waves which produce the sensation of 

 sound are excited by delivering a blow, or a series 

 of blows, to the air. If a bell be struck by a 

 hammer, its particles are thrown into a state of 

 vibration, as we may convince ourselves by lightly 

 applying our finger to the edge, and the vibrating 

 particles give rapid blows to the air, and thus pro- 

 duce sound-waves, which are propagated in all 

 directions. In this case, the sound is musical; but 

 if a table be struck by the hammer, its vibrations 

 cease almost immediately, and a single sound or 

 noise is the result. Thus we see that a musical 

 sound is produced when a definite noise is re- 

 peated often enough at equal intervals of time. 

 This statement may be verified by holding the 

 edge of a card so that it may be struck by a re- 

 volving toothed wheel. When the wheel revolves 

 slowly, a series of noises is produced ; but when it re- 

 volves so fast that the noises melt into a continuous 

 sound, a musical note is the result. Further, the 

 pitch of this note is higher, the faster the wheel 

 revolves ; and if the speed can be sufficiently in- 

 creased, the pitch becomes too high for the ear to 

 appreciate, and the notes become inaudible. There 



