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 O₁ is the image of pin hole S₁, M is a half silvered mirror, objective O₁, and eyepiece E construct an ordinary microscope, O₁, and O₂ are centers of a sphere with radius r and the distance of O₁, O₂ is r. When S₁ and S₂ are at symmetrical position with respect to M the image of S₁ is formed at S₂ in the two positions of sphere of O₁ and O₂ being reflected by the surface of the sphere. In this condition sharp image of S₁ can be observed in the field of microscope.
The measuring error of this apparatus is due to symmetry of S₁ and S₂ 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 CS₁ and CS₂ 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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The operating principle and the error of an interferometer of which reflecting mirrors are a pair of front-surface mirrors mounted to form a right angle, were discussed analytically. It was found that when the interferometer was used for the measurements of length by the fringe countiug method, the following adjustments were most important
(1) the positions and dimensions of entrance apertures of phototubes.
(2) the linearity of the locus of the moving mirror pair and
(3) the angle between the intersection line of the moving mirror pair and the direction of incident parallel light.

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