For minerals which have very strong double refraction, as zircons, so that the interference colors are of the higher orders, it is advisable to use the method of testing with a quartz wedge.[57] If a wedge is inserted between crossed nicols and its _c_ direction inclined at 45° to the planes of vibration of the nicols, then successive interference colors will be seen commencing with the gray of the first order and passing through the colors as shown by a color chart. If now the crystal section lies with its vibration directions also in the diagonal position, the color of any portion of the quartz wedge will be changed where it covers the section, the new color being that of a thicker part of the wedge if the _c_ direction in the section lies under the _c_ direction of the wedge. Also where the wedge overlaps the section the displacement of the color fringes in the section will be towards the thin edge of the wedge. On the other hand the new color will be that of a thinner part of the wedge when the _a_ direction in the section lies under the _c_ direction of the wedge. In this case the displacement of the color fringes will be towards the thick part of the wedge. This method is very convenient when the crystal section has in any place sloping edges, showing prismatic color fringes. The movement of these fringes (as previously described) is towards the thin edge of the wedge when the vibration directions correspond and away from this edge when they do not correspond. If the wedge when inserted finally “compensates” the color of the crystal section (_i. e._, practically produces an absence of color, darkness), then the a in this section must lie under the c in the wedge, as the effect of the wedge has been to continually thin the section. DETERMINATION OF ORDER OF INTERFERENCE COLOR. This can be determined by use of a quartz wedge or a v. Federow mica wedge.[58] Have the vibration directions of the given section in the diagonal position, then gradually insert the wedge between the crossed nicols so that the corresponding vibration directions in the section and wedge are crossed, that is so that the colors are run down until finally dark gray or black is obtained. Count the number of times the original interference color reappears, if _n_ times, then the color is a red, blue, green, etc., of _n_ + 1 order.[59] It is often easier to insert the wedge until compensation of the color is obtained; then as the wedge is pulled out the direct count of the recurrence of the original color will give the order of that color. METHOD OF MEASURING THE STRENGTH OF THE DOUBLE REFRACTION BY VON FEDEROW MICA WEDGE.[60] “The wedge[61] consists of fifteen superposed quarter undulation mica plates, each about two mm. shorter than the one beneath it and with their directions of vibration parallel. The series is mounted on a strip of glass and covered with a cover-glass.