Optics : a manual for students / by A. S. Percival.

  • Percival, A. S. (Archibald Stanley), 1862-
Date:
1899
Catalogue details

Licence: Public Domain Mark

Credit: Optics : a manual for students / by A. S. Percival. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    make this more clear. Let S represent the source of light emitting light of a wave-length (X) S I. Then if T be the undulation period, after the lapse of T the phase of S will occur throughout the circle of particles denoted by I. For instance, a crest will be formed at S at periodic intervals of time T. Similarly crests will be formed at periodic in- tervals of space X. The circles I, II, III will represent crests. Now, according to Huygens' principle, each particle in these circles will set up a new and independent wave system. Let us consider circle II, which will have been formed by S in the time 2T. In the time ST a crest due to S will have been formed at III. Meanwhile each particle in II will have set up an independent disturbance which will have extended in all directions a distance X. It will be seen that the external envelope common to all these small circles is continuous with III. The internal envelope is also continuous with I. Therefore the crests clue to these inde- pendent wave systems originating from II correspond to two wave-crests (I and III) originating from S. In a similar way it can be shewn that all the crests originally formed by S are reinforced by the new and inde- pendent wave systems that are originated by each particle on these crests. T Now in time ^ S will be in the trough of a wave, therefore in time 2T this trough will have extended 1J\ outwards. In time ST all the particles in the circle will have set up a trough-disturbance which will have extended a distance X from the circle It will be seen that the external envelope of these elementary trough-dis- turbances will be continuous with the circle 2-J-, representing the troughs formed by S in time ST. The internal envelope
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