684 REPORT OF NATIONAL MUSEUM, 1897. 



the surface of the second medium, changes its direction or is refracted. 

 When light passes from a rarer to a denser medium, it is refracted 

 toward the perpendicular; if from a denser to a rarer medium, it is 

 refracted away from the perpendicular. 



These conditions are universally true, and may be summarized as 

 folh)\vs: When light passes from a denser to a rarer medium, the angle 

 of incidence is less than the angle of refraction. When light passes 

 out of a rarer into a denser medium, the angle of incidence is greater 

 than the angle of refraction. The latio existing between the sines of 

 the angles of incidence and refraction is called the index of refraction. 

 This is constant for the same substance. 



The above assumes the existence of only one refracted ray; but there 

 are sometimes two refracted rays, whence it follows that an object seen 

 through such a medium appears double. Crystals which possess this 

 peculiarity are said to be double refracting. It is possessed to a greater 

 or less degree by all crystals which belong to systems of crystallizatiou 

 other than the isometric. 



Uniaxial and biaxial crystals. — In all double-refracting crystals there 

 is one direction, and sometimes a second, possessing the following prop- 

 erty: When a point is looked at through the crystal in such a direc- 

 tion, it does not appear double. The lines iixing these directions are 

 called optic axes. 



A crystal is uniaxial when it has one optic axis; that is, when there is 

 but one direction along which a ray of light can pass without being 

 doubly refracted. A crystal having two such axes is called biaxial. 

 Further, of the two parts into which the incident ray is divided on 

 entering a uniaxial crystal, one is called the ordinary and the other the 

 extraordinary ray. The one follows the law of single refraction; the 

 other, except under certain conditions, does not. The magnitude of the 

 refractive indices of these two rays always differ for the same crystal, 

 and should that of the ordinary ray exceed that of the extraordinary 

 ray the crystal is negative uniaxial; should the contrary be the case 

 the crystal is positive uniaxial. 



DIFFRACTION OF LIGHT. 



If a beam of light be made to pass through a narrow slit and by the 

 edge of an opaque body and to fall upon an appropriately placed screen, 

 the shadow of such a body will be divided into a series of light and 

 dark parallel bands, and the rays are diffracted. These bands or 

 fringes are due to the mutual reaction or interference of the adjoining 

 waves of light. Models 1 and 2 are of waves of light. The third model 

 shows the waves of 1 and 2 neutralized by superposition and interfer- 

 ence of two e(pial systems, the raised part of one wave fitting into and 

 making smooth the hollow of the other. The next iigure represents the 

 solar spectrum, showing the dark lines, called Fraunhofer's lines, which 



