The clinical use of prisms : and the decentering of lenses / by Ernest E. Maddox.

  • Maddox, Ernest E. (Ernest Edmund), 1860-1933.
Date:
1893
Catalogue details

Licence: Public Domain Mark

Credit: The clinical use of prisms : and the decentering of lenses / by Ernest E. Maddox. 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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    perpendicular* movements of the water (except in polarised light), but in all directions at right angles to the axis of the ray. Moreover, while the waves in a pond travel along the surface only, light travels in all directions from its source, so that we have to do with three dimensions instead of two. When light is generated at a point, it tends to travel out- wards from that point, not as we have seen, in an ever-widening circle, like the waves of a pond, but in an ever-increasing sphere (unless hindered, as it always is, in some directions by bodies in its way which absorb, refract, or reflect it). The waves in a pond are enlarging circles, those of light form enlarging spheres, and in each we can speak of a wave-front. The motion of the waves is always in a direction perpendicular to the wave-front,-|* a fact which, if grasped, will help us much. A moment's reflection will remind us that this is true * It is true that the motion of the particles of water is not quite so simple as this, since they describe curves, sometimes even ellipses or circles, which, being parallel to the direction of propagation, spoil the illustration for light, where the circular or eUiptical motions of the particles of atther are perpen- dicular to the direction of propagation. t I do not trouble the reader by explaining ivhy this is so. It really depends on the principle of Huygens, that the disturbance at any point of a wave-front is the resultant of the separate disturbances which the different portions of the same wave-front, in any one of its earlier positions, would have occasioned if acting singly.