The radius of curvature of Rockwell diamond indenter is one of the most important factors for the accurate estimation of hardness, but an adequate method has not been discovered. A new apparatus was designed applying the method of microscopic collimation. The principle of this method is shown in Fig. I in which O
1 is the image of pin hole S
1, M is a half silvered mirror, objective O
1, and eyepiece E construct an ordinary microscope, O
1, and O
2 are centers of a sphere with radius
r and the distance of O
1, O
2 is
r. When S
1 and S
2 are at symmetrical position with respect to M the image of S
1 is formed at S
2 in the two positions of sphere of O
1 and O
2 being reflected by the surface of the sphere. In this condition sharp image of S
1 can be observed in the field of microscope.
The measuring error of this apparatus is due to symmetry of S
1 and S
2 and depth of distinct vision of microscope. Concerning these factors analysis and experiments were carried out and the following results were obtained : In order to keep the measuring accuracy within I micron, the defference of CS
1 and CS
2 must be less than 5.6 mm when the magnification of objective is × 40.
The measuring accuracy concerning depth of distinct vision is illustrated in Table 2 for some combinations of objectives and eyepieces in which we can see the error is less than 0.4 micron for the combination of objective × 40 and eyepiece × 10.
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