278 



ALBERT JOHANNSEN 



Fig. 2 



Place the stylus of the planimeter at the top of the field of 

 view and on the vertical cross-hair (Fig. 3, A). The writer 

 replaces the ordinary stylus by a heavy wire bent as shown in 

 Fig. 2, so that the end of the planimeter bar will not interfere 

 with a view of the end of the needle point. Adjust the length of 



the planimeter bar so that when the stylus 



I is moved around the semicircle (Fig. i), 

 that is, half the field of view, it will 

 register exactly unity (10 or 100). This 

 adjustment, of course, is not necessary, 

 but by making it all readings are in per- 

 centage values without conversion; otherwise it is necessary to 

 reduce the sum of the readings to 100. 



Record the reading of the planimeter or set it to zero by rotating 

 the wheel. If the bar has not been set to read 100 for the field of 

 view, move the stylus around the periphery of the field in the 

 proper direction, right or left, to record additions, and read again. 

 The difference will give the area of a single field. Again set the 

 planimeter at zero, and, beginning at A, move the stylus along 

 the straight-edge to the periphery (Fig. 3, 5) of the first grain of 

 the particular mineral chosen for the first determination, say 

 biotite in granite. Trace the outline 

 of this grain and remember always to 

 move the stylus in the same direction of 

 rotation. It will be necessary in the 

 first readings to lift the stylus point 

 on top of the straight-edge to record 

 the left-hand portion of the mineral. 

 Having returned to the point B, with- 

 out reading the planimeter move along 

 the straight-edge to C on the periphery 

 of the next grain of biotite lying on the 

 line of the vertical cross-hair, and move 



around it in the same way and in the same direction back to the 

 point C. If the mineral of the kind being measured lies in 

 the center of the circle (Fig. 3, Z)), it is always to be measured in 



Fig. 3 



