The Optical Magic Lantern Journal (August 1894)

The Optical Magic Lantern Journal and Photographic Enlarger. 133 to the feature of progression as it would present itself in a section taken through a sound sphere in any plane that would intersect the centre of the sphere at which is located the source of sound. The mechanical slide shown in Fig. 1, when projected, is capable of producing on the screen a series of concentric rings of light and shade, representing the condensations and rarefactions of a succession of sound waves, and these waves, beginning at the centre, constantly enlarge in circumference until they disappear at the periphery of the disc. produced by means of two thin metal discs arranged to revolve on the same axis, and each provided with a spiral slot extending from centre to periphery, the slot of one disc being oppositely arranged with respect to that of the other disc. One disc is secured to a sleeve which fits on a stud supported by a fixed bar extending across the opening of the slide. The other disc turns on the sleeve. The sleeve and the dise which turns upon it are each provided with a small pulley. One of these pulleys is slightly larger in diameter than the other, so that when the two discs are projected and revolved rapidly in the same direction, one turning at a very slightly increased speed causes the points of intersection of the spiral slots to move outwardly and thus produce on the screen a series of light rings, which increase in diameter like mill pond waves. To cause the light rings Fig. 2. Interference. and intervening dark rings to blend into each other, the slide is:thrown a little gut of focus: To show interference of sound waves two images of the slide may be projected, one being Fig. 3. This effect is. ! the two parts of | slightly different } Arrangement for projecting two images on the slide. superposed on the other as shown in Fig. 2. This is easily done by arranging at a suitable -angle in front of the lantern objective a series of ‘glass plates, such as are found in a glass plate polarizer, as in Fig.3. A portion of the beam is transmitted, forming one image on the screen, and a portion is reflected upward and intercepted by a mirror which throws it upon ‘the screen, forming a second which may be made to coincide with the first, or it may be made to overlap the first imageso as to produce the interference effect shown in Fig. 2. In this case the centres or wave sources are separated more than the semi-diameters of the discs, and the interfering waves approach each other from opposite directions. In Fig 4. are shown, diagrammatically, superposed wave discs with centres one wave length apart. The waves’ ‘crests "’ coincide, and re-enforcement along a line joining the two centres is the result. If the centres were half a wave length apart, the ‘“‘erests’’ would alternate and one set of waves would neutralize the other. In Fig. 5 are shown diagrammatically twe discs of differen: size produced by § dividing the beam before it passes through the objective, projecting the beam witb objectives of Fig. 5. Beats. power. In this case, owing to the difference | in the size of the discs, the relative velocities of the wave ringe differ, so that the waves of one serles Overtake the waves of the other series at a, thus illustrating the phenomenon of beats. "| ———105 EXPERIMENT..— Write with dilute nitrate of silver, which when dry will be entircly invisible, hold the paper over a vessel containing sulphate of ammonia and the writing will appear very distinct, the letters shining with the metalic z brilliancy.of silver.