McNair — A Method in Teaching Optical Mineralogy. 295 



change occurring at the ray axis, which is close to the nose of 

 the hyperbola of the picture. 



Proceeding now from the center of the figure at right angles 

 to the axial plane, the ellipses undergo no such change, the 

 major axis now remains constant at 2a while the minor axis 

 grows smaller, reducing from 25 toward 2c. The major and 

 minor axes of these ellipses represent the speeds of the two 

 disturbances vibrating respectively in the planes of these axes. 

 If we call the difference of these speeds A, as is convenient, it 

 is easily seen that A near the nose of the hyperbola is zero, 

 and that from this spot it increases toward the center of the 

 figure, while at right angles to the trace of the axial plane it 

 increases away from the center. Outside of the hyperbola and 

 away from the center it increases in numerical value but is 

 negative. 



Suppose now that the quartz wedge is pushed over the 

 plate, thin edge first, with its vibration directions as indicated 

 by the ellipse drawn in its corner. It will readily be seen that 

 this ellipse lies parallel to that for the central ray of the cone, 

 and to all others inside of the hyperbola. The faster ray in 

 the plate will also be the faster in the wedge, and the slower 

 in the plate will be slower in the wedge. The effects of 

 plate and wedge will be added. Now the location of the color 

 bands depends directly on the value of A, hence for a given 

 position of the wedge a particular band will be found where 

 the combined A of wedge and plate equals the A of the plate 

 alone at the original location of the band before the super- 

 position of the quartz wedge. As the wedge is advanced the 

 combined A at any point of the plate is increased continuously, 

 while any particular value of the combined A necessarily shifts 

 toward that spot in the plate where A is least. Therefore, as 

 the wedge is pushed over, the color fringes will travel from 

 the center toward the nose of the hyperbola. Likewise, on 

 the line at right angles to the axial plane they must travel in 

 toward the center. 



Outside of the hyperbola, however, the ellipses are in crossed 

 position, and the slower ray in the plate is now the faster in 

 the wedge. A difference of effects results, and a given color 

 fringe will now travel toward a position in the plate where A 

 is numerically greater. Therefore, outside of the hyperbola 

 the color fringes will travel away from the nose. Of course, 

 a reversal of the quartz wedge, either by turning it over to 

 reverse the position of its ellipse or pushing it thick end first 

 instead of thin, reverses the travel of the fringes. For a neg- 

 ative crystal, the travel would be in each case in the opposite 

 direction. 



There are approximations in the foregoing which have not 



