To test the flatness of an optical fiat, a simple optical system, combined with the Fabry Perot etalon, has been [1evise[1, which affords sharp contour lines of elevation (or depression) in the image of the flat. When the fiat to be tested is held oblique to that to be referred, the elevation is indicated by the deviation of contour lines which are to be straight, parallel and equidistant if the flat is perfect, Analyzing the contour lines, the flatness has been measured with the accuracy of 1/200λ When the flats are held in parallel, only one contour line appears and in order to displace the line, the pressure of air in the etalon has been varied. In this case, the flatness can be surveyed without any analysis. This optical system is available to measure a minute displacement or a small change of an angle. In conclusion, the writers wish to express their best thanks to Dr. J. Okubo, ex-super-intendent professor of this institute, for his continued interest and encouragement in this work
