Effect of Bodies on Light and Heat. 397 



of the magnetic current. Another experiment is performed by using in- 

 stead of the homogeneous glass, a peculiar crystal made by taking pieces 

 of crystalline quartz plate, one laevogyre and the other dextrogyre, each 

 having one straight edge by which they are cemented together. Such a 

 crystal placed between the nicols while they are parallel will produce two 

 different colors, one on each side of the cemented line. But by turning 

 one of the nicols in azimuth, a position can be reached in which both 

 halves of the image transmitted will be of the same color. If now a 

 magnetic current be passed through the quartz crystal, each half will 

 give an independent color and the two will be complementary of each 

 other, as red and green ; and if the current be reversed by changing the 

 polar connections, the two halves of the colored image will exchange 

 places, the red half becoming green and the green half, red. The effect 

 is to cause a rotary twist in the plane of vibration the same as if the 

 analyzer were turned in azimuth. These effects of magnetism endure 

 only while the current is in action. 



From what has gone before, it is fair to conclude that it is the variety 

 of the forms of matter with which the waves of light come into contact 

 that causes the variety in the phenomena. As in the case of the lower rates 

 of vibration producing sounds which are" modified, reflected or absorbed 

 according to the form of the body with which the waves come into col- 

 lision, so these higher tones called light are subject to the same ac- 

 cidents. When we speak of form, not merely the external configuration 

 is meant, but the molecular composition also, which has much to do 

 with the effects whatever be the rate of vibration. This is especially 

 true of the phenomena of light just considered. Double refraction is 

 always accompanied by polarization. But double refraction cannot 

 happen to light passing through a homogeneous medium, and by this 

 term is meant a medium whose particles are similar in structure and 

 relative position throughout the mass. There are other crystals whose 

 molecules are arranged in a symmetrical manner about lines running in 

 some particular direction only. This is the case with Iceland spar as 

 stated above. The shortest diagonal of the natural crystal is this ' ' optic 

 axis," every line running in that direction, i. e., parallel with the optic 

 axis, passes through tracts symmetrical on all sides at any one place. 

 This is also true of ice, the optic axis being perpendicular to the plane 

 of freezing. Light sent in this direction through ice or any other crys- 

 tal, passes as if through a homogeneous body and it goes staight with- 

 out being refracted. It is retarded if the light has come into it from a 

 rarer medium, but this retardation manifestly can only bend the ray 

 when it enters the crystal at an angle to the optic axis. In the case of 

 double refraction the one ray is split into two, one of which is slower 

 than the other ; one half meets with a greater resistance than the other. 



