. The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology. Biophysics. 39S PHYSIOLOGICAL PHYSICS. ichap. xxx. move round the other which is stationary. This phenomenon is due to double refraction, and was dis- covered in 1669 by the Professor of Geometry in Copenhagen, Erasmus Bartholinus. The explana- tion is that when a ray of light enters such a crystal it is split up into two, and the two rays travel through the crystal, with different velocities. One ray is retarded more than another, that ray is,
. The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology. Biophysics. 39S PHYSIOLOGICAL PHYSICS. ichap. xxx. move round the other which is stationary. This phenomenon is due to double refraction, and was dis- covered in 1669 by the Professor of Geometry in Copenhagen, Erasmus Bartholinus. The explana- tion is that when a ray of light enters such a crystal it is split up into two, and the two rays travel through the crystal, with different velocities. One ray is retarded more than another, that ray is, consequently, refracted more than the other, and when the rays issue from the crystal they do not unite, but are dis- placed from one another, so that a double image is pro- duced (Fig. 178). One ray travels through the crystal just as it would do through a o plate of glass, being refract e d in the ordinary way. This is the ordinary ray, and is the ray which gives the stationary image. * o The other ray, which suffers the smaller degree of retardation, is called the extra- ordinary ray, and is the ray which gives the movable image when the crystal is rotated. To this ray the or- dinary laws of refraction do not apply. Both rays are of equal brilliancy. An explanation of the different course of the two rays is offered by supposing that doubly refractive crystals are not equally elastic in all directions, and consequently vibrations in different directions are subject to differences in retardation. There is, however, always one direction in which a ray of light will be transmitted without double refraction. This direction is that of the optic- axis of the crystal. Crystals that have more than. Fig. 178.—Iceland Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original M'Gregor-Robertson, J. (Joseph), 1858-1925
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