Six lectures on light : delivered in the United States in 1872-1873 / by John Tyndall.

  • John Tyndall
Date:
1882
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Licence: Public Domain Mark

Credit: Six lectures on light : delivered in the United States in 1872-1873 / by John Tyndall. 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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    LECT. circle at n. When it is incident along m' E there is also refraction at E, the beam striking the point n'. From the ends of the incident beams, let the perpen- diculars m o, m' o' be drawn upon B D, and from the ends of the refracted beams let the perpendiculars p n, p' n' be also drawn. Measure the lengths of o m and of p n, and divide the one by the other. You obtain a certain quotient. In like maimer divide m! or by the corresponding perpendicular p' n'; you obtain Fig. 5. 15 the same quotient. Snell, in fact, found this quotient to be a constant quantity for each particular sub- stance, though it varied in amount from substance to substance. He called the quotient the index of refraction. In all cases where the light is incident from air upon the surface of a solid or a liquid, or, more gener- ally still, when the incidence is from a less highly refracting to a more highly refracting medium, the reflection is partial. In this case the most powerfully reflecting substances either transmit or]absorb a portion of the incident light. At a perpendicular incidence