Journal of Applied Microscopy. 



21 



the cross-hairs. Push in the quartz 

 wedge with its axis of least elasticity 

 parallel to the line joining the hyperbola 

 eyes. If the hyperbola eyes open and 

 move toward the center of the lemnis- 

 cate figure, the mineral is positive. 



Push in the quartz wedge with its axis 

 of least elasticity perpendicular to the 

 line joining the hypei"bola eyes. If these 

 eyes open and move toward the center 

 of the lemniscate figure, the mineral is 

 negative. 



Of course, if in either case the eyes 

 contract and move outwards, this is 

 proof, when the axis of least elasticity of 

 the quartz wedge is perpendicular to the 

 line joining the hyperbola, that the min- 

 eral plate is positive; but if they move 

 outward when the axis of elasticity is 

 parallel to the chosen line, the mineral 

 plate is negative. 



This method is less satisfaotory in 

 practice than the one where the eyes 

 open and move inwards. 



(b) The above method given in our text 

 books can be supplemented by one that 

 can be employed in numerous cases 

 when both of the hyperbola eyes can not 

 be seen, but only one of them or only the 

 lemniscate arcs. In either of these cases 

 the positive or negative character of the 

 mineral plate can be ascertained; if one 

 can determine the position of the line 

 joining the hyperbola eyes or optic axes, 

 by the form of the interference figures, 

 by the position of the larger arm of the 

 cross, or by any other means. When 

 this direction is observed, place the arcs 

 so that the direction of tne line joining 

 the hyperbola vertices shall be perpen- 

 dicular to, or bisect, them; also have this 

 line make an angle of forty-five degrees 

 with the cross-hairs, as before, Push in 

 the wedge with its axis of least elasticity 

 perpendicular to the arcs, or parallel to 

 the line joining the hyperbola eyes. If 

 the lemniscate arcs move in towards the 

 center of the field with their convex side 

 forwards, the mineral is positive. 



Push in the wedge with its axis of least 

 elasticity tangent to the arcs, or perpen- 

 dicvilar to the line joining the vertices. 

 If the arcs then move in with their con- 

 vex side forwards, the mineral is nega- 

 tive. If the arcs move out^^ards with 

 their concave side forwards, the mineral 

 in the first position of the wedge is nega- 

 tive, and in the second position positive. 



(c) If the distance between the hyper- 

 bola eyes is not so great but that they 

 lie within the field of view, the mica and 

 gypsum plates can both be employed to 

 determine the positive and negative 

 characters when the lemniscate figure 

 is placed as before, with the line joining 

 the hyperbola eyes forming an angle of 

 forty-five degrees with the cross-hairs 

 of the eye-piece. Insert either the mica 



plaJte with its axis of least elasticity par- 

 allel to the chosen line, or else insert the 

 gypsum plate with its axis of least elas- 

 ticity perpendicular to the chosen line. 

 With either plate in this position, the 

 arcs on one side of the hyperbola eyes 

 will enlarge and those on the other side 

 contract. If the arcs that lie on the in- 

 side of the eyes, or nearest the center of 

 the figure, enlarge, and those on the out- 

 side contract, the mineral is positive. On 

 the other hand, if the arcs nearest the 

 center contract and the outside arcs ex- 

 pand, the mineral is negative. This 

 method can be used with plates that 

 have too great an axial divergence for 

 their determination when the unsym- 

 metrical cross is placed with its arms 

 parallel to the cross-hairs. 



III. THE CHROMATIC SCALE. 

 Many students find it difficult to follow 

 the color-scales given in most text-books 

 of petrography owing to the numerous 

 subdivisions of the scales. This diffi- 

 culty can be obviated in part by each 

 student making for himself a color-scale 

 suited to his eyes and experience. It is 

 found that many students mistake their 

 ignorance of the names of color-tints for 

 color-blindness. The scale is made by 

 placing the quartz wedge on the stage of 

 the microscope with the nicols crossed. 

 Then push the wedge with its thin end 

 forward through the field of view of the 

 microscope. Note the colors as they rise 

 in the scale, as the successively thicker 

 portions of the wedge pass in view. The 

 scale thus noted will be suited to the 

 wedge employed and to the student using 

 it at that stage of his experience. The 

 operation can be repeated with the nicols 

 parallel if desired. 



IV. SECTION AND PLATE. 



I have found it convenient in practice 

 to distinguish the terms "section" and 

 "plate" in the microscopic study of min- 

 erals and rocks as follows: The term 

 "section" is employed to indicate the en- 

 tire maps of the rock or mineral that is 

 carried by the glass slide used on the 

 stage of the microscope. The term 

 "plate" is introduced to designate a par- 

 ticular section or slice of mineral or 

 other substance that forms a part of the 

 rock or general mass carried by the glass 

 slide. A "section" is composed of 

 "plates." A rock "section" is usually 

 made up of many mineral "plates," either 

 held together by intercrystallization or 

 by some cementing material, which ma- 

 terial in its turn lies in an irregular 

 "plate" or in "plates." 



"Plate" is never the equivalent of 

 "section," unless a single "plate" of one 

 mineral forms the entire "section." 



Michigan College of Mines, Houghton, 

 Michigan, December 16, 1897. 



