The theory of light / by Thomas Preston.

  • Preston, Thomas.
Date:
1901
Catalogue details

Licence: In copyright

Credit: The theory of light / by Thomas Preston. Source: Wellcome Collection.

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

    565/616 (page 541)
    hence rj/^ varies inversely as —that is, the smaller the drops the farther apart are the supernumerary bows ; and this explains why they are only well defined near their summits, for as the drops descend they grow in magnitude, and consequently the lower portions of the bow deviate less and less from the geometrical position. The same reasoning applied to the second principal bow shows that it also is accompanied by spurious bands situated at its outer edge and best defined at its summit. This bow also deviates from the geometrical position, its radius being somewhat greater than that assigned by the elementary theory. 326. Miller's Experiments.—Airy's theoretical results have been confirmed by the experimental investigations of Mr. Miller.^ A pencil of sunlight was admitted in a horizontal direction through a narrow vertical slit, and fell upon a thin vertical jet of water. View- ing the stream through a telescope (or with the naked eye) portions of the primary and secondary bows, and a large number of the spurious bands could be seen forming a series of vertical coloured fringes arranged side by side. The diameter of the water jet was about '022 of an inch. The mixture of colours rendered it difficult to fix upon the brightest parts of the bands. The mean of eight obser- vations was as follows :— Radius of brightest part of primary bow 4]° 32' \ ,, ,, ,, first spurious bow 40° 27' / Radius of brightest part of secondary bow 51° 58' 1 ,, first spurious bow 53° 57' J ' According to the elementary theory we should have Radius of brightest part of primary bow 41° 53'9 \ ,, secondary bow 51° 12''9 j In conclusion it may be remarked that the light of the rainbow is Light partially polarised. This polarisation was noticed by Biot as early as 1811, and is to be expected as a consequence of the reflection and refraction suff*ered by the light in the drops of rain (Art. 173). The extent to which the light of any bow is polarised may be easily cal- culated by resolving the incident vibration into two components—one polarised in the plane of incidence, and the other perpendicular to it, and then applying the formulae of Arts. 208, 209.