20 



Journal of Applied Microscopy. 



Some Methods of Determining the 

 Positive or Negative Character 

 of Mineral Plates in Converging 

 Polarized Light with the Petro- 

 graphical Microscope. 



i)E. M. E. Wadsworth. 



In giving instruction in the use of the 

 petrographical microscope as a polari- 

 scope, I have found a few directions of 

 value to my students — directions which I 

 do nO't remember of having ever seen 

 publislied. Since by varying the powers, 

 the petrographical microscope can be 

 used with mineral plates of any standard 

 thickness, the directions here given can 

 be used with the ordinary polariscope 

 plates, as well as those thinner ones pre- 

 pared expressly for use with the micro- 

 scope. 



I. UNIAXIAL MINERALS. 



When the mineral plate shows the 

 common uniaxial cross in converging 

 light, its positive or negative character 

 can be ascertained by means of the gyp- 

 sum plate or quartz wedge, as well as by 

 the ordinary mica plate. 



(1) Use of the Gypsum Plate.— Examine 

 the mineral plate, which, in converging 

 polarized light between crossed nicols, 

 shows a dark cross or part of a cross 

 with or without colored rings or arcs. 

 Insert the gypsum plate in the slot in the 

 body of the microscope, above the ob- 

 jective. The cross is then resolved into 

 colored hyperbolas. The central portion 

 is red, terminated on the ends with yel- 

 low, and bordered on the side by blue. 

 If the blue thait borders the red lies on a 

 line parallel to the axis of least elasticity, 

 the mineral is positive; but if it lies on 

 opposite sides of this line, the mineral is 

 negative. The gypsum plate is often 

 more satisfactory in its use than the 

 mica plate for these determinations. 



(2) Use of the Quartz Wedge.— Insert 

 the quartz wedge thin end forward. 

 When the wedge is gradually pushed in, 

 the cross resolves itself into colored arcs 

 that cross the field of view from two 

 opposite sides of the field, and pass out 

 of sight on the other two sides. These 

 arcs follow each other in succession as 

 the wedge is pushed in. If these colored 

 arcs advance towards the center of a 

 line parallel to the axis of least elastic- 

 ity, the mineral is positive; but if they 

 march toward the center from opposite 

 sides of that line, the mineral is negative. 



The use of the quartz wedge is less lia- 

 ble to error than either of the preceding, 

 and beside it can be used in many cases 

 where the others give no results. 



(a) If the uniaxial plate is cut so that 

 it shows arcs of rings, its positive or 

 negative chariicter can be determined by 

 placing the arcs so a line perpendicular 



to them shall make an angle of forty-five 

 degrees with the cross-hairs. By use of 

 the quartz wedge, colored arcs or rings 

 can often be brought into the field, when 

 otherwise none are seen. Push in the 

 quartz wedge with its axis of least elas- 

 ticity tangent to the arcs. If the rings 

 then move outwards with their convex 

 side forward, and, in time, a black or 

 partially black arc appears, the mineral 

 is positive; but if the arcs move with 

 their concave sides forwards, the min- 

 eral is negative. 



As a check against any error, turn the 

 wedge over and push it in, so its axis 

 of least elasticity will be perpendicular 

 to the arcs. If then the arcs move with 

 the concave side forward, the mineral is 

 positive; but if they move with the 

 convex side forwards, and a black or 

 partially black ring or rings show, the 

 mineral is negative. 



(b) A uniaxial plate cut parallel to the 

 vertical axis can have its positive or 

 negative character shown in converging 

 polarized light as follows: Place the 

 plate at an angle of forty-five degrees 

 with the cross-hairs so as to show the 

 colored arcs or imperfect hyperbolas. 

 Push in the quartz first with its axis of 

 least elasticity perpendicular to the ver- 

 tical or optic axis of the plate. If on 

 pushing along the quartz wedge a dark 

 hyperbola is seen to pass over the field, 

 the mineral is positive. Again, push in 

 the quartz wedge w^ith its axis of least 

 elasticity parallel to the vertical axis of 

 the plate. If then a dark hyperbola is 

 seen to traverse the field, the mineral is 

 negative. 



II. BIAXIAL MIXERALS. 



In order to render intelligible the di- 

 rections later given, there is here stated 

 the method published in the text-books 

 for determining the positive or negative 

 character of a biaxial mineral plate. 



If a line of extinction of a biaxial plate 

 properly cut is placed parallel to one of 

 the cross-hairs, it shows a cross with 

 unequal arms; but if the line of extinc- 

 tion makes an angle of forty-five degrees 

 with that cross-hair, it shows two dark 

 hyperbolas, whose vertices or eyes mark 

 the position of the vertical axes. Ac- 

 companying the cross and hyperbolas 

 are colored lemniscate figures. Often- 

 times the hyperbolas are wanting and 

 only the colored lemniscata can be seen; 

 but by the insertion of the quartz wedge, 

 the hyperbolas can frequently be brought 

 into the field. 



(a) The positive or negative character 

 of this biaxial plate can then be deter- 

 mined by placing the plate on the stage 

 in such a position that a line joining the 

 hyperbola eyes, or bisecting the lemnis- 

 cata througii their longest direction, shall 

 form an angle of forty-five degrees with 



