INTRODUCTION. II 



upper Nicol or analyzer be turned alwut 90°, the field of the microscope will become 

 light, because the light passing through the lower Nicol or polarizer passes through 

 the analyzer, also, under the same conditions. If now, again, the amorphous section 

 is introduced, the field will be still light. In general, place the Nicols as we will, the 

 light will not be modified by placing an amorphous substance between them. More- 

 over, an amorphous substance shows no definite cleavage lines, or no crystalline out- 

 line. In New Hampshire we live in a region of old crystalline rocks ; and hence, with 

 some rare exceptions, we have but little to deal with amorphous substances. 



Isonict)-ic Crystals. Isometric crystals being developed symmetrically in each of 

 their three directions, the elasticity of the ether is the same in all directions : and 

 hence, in isometric crystals, light passes in all directions and planes with equal ease, 

 and this gives to them the same optical character as amorphous substances. Isometric 

 bodies in their sections can, however, commonly be recognized as crystals, since they 

 generally possess either a definite polygonal outline or cleavage lines. These bodies, 

 which possess simply the power of single refraction, are called isotropic. 



Tetragonal and Hexagonal Crystals. The case becomes quite different when any 

 other body except those mentioned is placed between the Nicol prisms. Tetragonal 

 and hexagonal crystals are not symmetrical in all directions ; and hence the elasticity 

 of the ether is different in different directions. It is either greater or less in the plane 

 of the vertical axis than it is in the plane of the lateral axes; and, if a beam of light 

 passes through a section of one of these crystals, which^is cut parallel to the vertical 

 axis, its vibrations will, in passing through the crystal, take place in these planes of 

 elasticity ; and, as the elasticity is greater in one direction than the other, that part of the 

 ray, the vibrations of which take place in the plane of greater elasticity, will be retarded 

 less than those that take place in the plane of least elasticity, which is at right angles 

 to the first ; hence the ray of light will emerge from the crystal having all its vibrations 

 reduced to two planes, and one of these sets of vibrations will be in advance of the 

 other by a certain amount, depending upon the nature of the substance and the thick- 

 ness of the section. In other words, the crystal in this direction is double refracting; 

 and the law may here be stated, that the light, by its entrance into any double refract- 

 ing section, is divided into two rays, each of which is polarized. The planes of vi- 

 bration of these rays are at right angles to one another ; and these planes corre- 

 spond to the directions of the greatest and least elasticity of the ether in the section. 

 Let us now suppose such a section to be introduced into the field of the microscope 

 while the Nicol prisms are crossed. If we place it so that the vertical axis of the crystal 

 is parallel to the plane of vibration of the light as it issues from the lower Nicol, the 

 light will pass through the crystal without further modification, since the plane of 

 greatest or least elasticity in the crystal section corresponds with the plane of vibration 

 of the light; and, as the light meets the crystal in one of the two planes in which it 

 can pass, the crystal does not alter it, and it is therefore, as before, cut off by the upper 

 Nicol, and the field remains dark. If, now, we revolve the table of the microscope a little, 

 so that the principal axis of the crystal does not correspond with the plane of vibration 



