determining the Hardness of Minerals. 407 



Apparatus for recording depth. — To the end of the beam on 

 its upper side is pivoted horizontally a ring which may be 

 inclined about the symmetry axis of the beam. In this tits a 

 circular plate, m, bearing a transparent Zeiss micrometer glass 

 (5 ram divided into 100 parts). The plate, m, is rotary in the 

 ring, so that the micrometer scale may be turned in any azi- 

 muth. This device is so adjusted that the micrometer scale is 

 visible in the field of the microscope at the point exactly 10 mm 

 from the axis of rotation of the diamond point : this is one- 

 sixth of the distance from the diamond axis to the beam pivots, 

 a, hence any downward movement at the diamond point is 

 magnified by 1/6 at the micrometer. Hence if the microscope 

 is focussed on the micrometer, before and after boring with 

 the diamond, the depth so measured by the fine adjustment 

 screw of the microscope will be 7/6 of the actual depth bored. 

 If, now, m be rotated until the micrometer scale stands at right 

 angles to the beam, and be then tipped gently, an inclination 

 may be found where, under a high power, only one line of the 

 micrometer scale is in focus at a time, and a downward focus 

 of precisely -01 mm or 10 microns (micromillimeters) is necessary 

 to bring the next lower line on the slope into focus. Con- 

 versely, if we focus on the lower line and allow the diamond 

 to bore its way down 10fi, the next higher line of the microm- 

 eter glass will come into sharp focus only when that depth is 

 reached. We thus have here an extremely sensitive measure 

 of depth. 



Method of using the apparatus. — The standard is first 

 adjusted to the microscope foot, so that the diamond is exactly 

 10 mm from the center of the stage (cross-hairs) as recorded on 

 the stage micrometer scale movement. The preparation is 

 fixed in the angle piece and clips on the stage. The beam be- 

 ing in perfect balance, it is locked : record is made of the read- 

 ings of i, i 1 and R. Equal weights are placed on each pan. 

 The mineral is centered to the cross-hairs of the microscope, 

 and a part of the mineral surface is selected for the test under 

 the low power, No. 2, whose long focus prevents the objective 

 from striking m. By the stage movement the mineral is 

 pushed 10 mm , when it is exactly under the diamond. The lat- 

 ter is lowered to the mineral surface until slight contact is indi- 

 cated by the movement of the indicator hand : it is checked 

 exactly at the point of contact — if anything a little above it. 

 The lock, Z, is released so that the diamond is actually resting 

 on the mineral, and the micrometer scale, m, is focussed under 

 objective (JSTo. 7) of the microscope, so that the lowermost of 

 two lines near the center of the field are in focus, with the 

 inclination arranged 10/jl to the scale division : ocular No. 3 is 

 most effectual, giving considerable spherical aberration which 



