The principles and methods of geometrical optics : especially as applied to the theory of optical instruments / by James P. C. Southall.

  • Southall, James P. C. (James Powell Cocke), 1871-1962.
Date:
1913
Catalogue details

Licence: In copyright

Credit: The principles and methods of geometrical optics : especially as applied to the theory of optical instruments / by James P. C. Southall. Source: Wellcome Collection.

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

    47/702 (page 15)
    uted to Euclid (300 B. C). On the other hand, the law of refraction is much more modern. Claudius Ptolem^eus, the great astrono- mer, who nourished during the reigns of the Antonines, published a treatise on optics ('Otttlky) Trpay^areia) in which he describes a number of experiments whereby he measured the angles of incidence and re- fraction, without, however, discovering the law. The next experi- ments along this line of which we have any record are those of Alhazen who died in Cairo in 1038; he repeated the experiments of Ptolem^eus, but added nothing to the previous knowledge of the matter. Kepler also made experiments, but was equally unsuccessful. The real dis- coverer of the law was Willebrord Snell, of Leyden, who announced it some time prior to 1626. It was first published by Descartes1 in 1637; who seems undoubtedly to have obtained it from Snell, although he failed to mention his name in connection with it. 18. In the case of Reflexion, it is obvious that the directions of the incident and reflected rays may be reversed, so that if PBR (Fig. 5) represents the path pursued by the light in going from P to R, under- going reflexion at the incidence-point B, then RBP will represent the path which the light takes in going from R to P under the same cir- cumstances, that is, via the incidence-point B. Experiment shows that the same rule holds good also for the ray refracted at B; so that if PBQ is the route followed by a ray in going from a point P in the medium (a) to a point Q in the medium (b), undergoing refraction at the incidence-point B, the same route will be pursued in the reverse sense QBP by a ray whose direction in the medium (b) is from Q towards B. And, hence, since sin a sin a! sinV = n°» smT = Uba' we have obviously, the relation: nab'Kba = I- (2) This general law of optics, known as the Principle of the Reversi- bility of the Light-Path, may be stated as follows: If a ray of light, undergoing any number of reflexions and refrac- tions, pursues a certain route from one point A to another point A', and if at A' it is incident normally on a mirror so that it is reflected 1 Rene Du Perron Descartes: Discours de la methode pour Men conduire sa raison et chercher laverite dans les sciences; plus la Dioptrique, les Meteores et la Geometrie (Leyden, 1637).