Effect of Bodies on Light and Heat. 391 



be called a conjugate plane. In every conjugate plane the separation of 

 the two rays by double refraction is at its maximum. Both rays in 

 such case have their index of refraction constant ( but different from 

 each other). The index of the ordinary ray is 1.6543, and 1.4833 for 

 the extraordinary. This difference of index explains why an object 

 seen by the ordinary ray appears nearer than by the other. See fig. 161. 

 The sine D L being shorter, the perpendicular D E is further to the 

 right nearer the normal, and the point B (or apparent position of the 

 object D ) is nearer to the point A, the position of the eye. Double re- 

 fraction is always accompanied by Polarization more or less complete. 

 In the Iceland spar crystal, truncated perpendicular to the optic axis, 

 the extraordinary ray is bent further from the normal than the ordinary 

 ( as if repelled ). ( When the extraordinary index is greater than the 

 ordinary, the crystal is called positive, when less it is negative. ) In 

 some other crystals the position of the rays is reversed, the extraor- 

 dinary one being nearer the optic axis, which is parallel with the nor- 

 mal. In crystals of Iceland spar all the vibrations are quenched except 

 in two azimuths, or are reduced to two azimuths, at right angles to 

 each other, the vibrations of the ordinary- ray being in one azimuth 

 only, and those of the extraordinary ray exclusively in the other. It is 

 important to get a clear idea of the polarization of light. It has been 

 stated that a great many bodies are of such molecular structure that 

 while they allow light to pass through them, they cause it to pass by 

 boring through in a spiral manner. This was never known until polar- 

 ization was found out. Let us illustrate by calling a beam of light a 

 pine saw-log, which we are to imagine is to be sawed up into boards. 

 A pine log may be divided into boards by vertical seams, each board 

 being of even thickness but of different widths, the one in the middle 

 being the widest and the others diminishing in width on each side. 

 But with the beam of light it is different. All the boards to be sawed 

 out of it are of the same width, and all maximum, and they are not 

 parallel with each other. Only one of them is vertical, right out of the 

 middle of the beam. Another is horizontal right through the middle of 

 the beam in that direction and so at right angles to the first. And it is 

 of equal width with the first. The other boards are to be sawed at all 

 the various oblique angles between these first two, every board to be 

 sawed directly in the middle of the log, and to be of the same maximum 

 width. The waves of the material of this log are supposed to corres- 

 pond with these different boards, the vibrations being always across the 

 middle of the beam in the plane of some board. Consequently, they 

 are said to be in all azimuths or directions from the center of the beam. 

 The waves, as said before, must be conceived to be like those of a 

 string stretched in the direction of the long axis of the beam. 



