After outlining the methods that have been employed for measuring the spherical aberration of microscope objectives, a new method is described in which a thin metal plate having a small hole in it is mounted between the objective and the microscope tube in such a way that the hole may be traversed across the middle of the objective aperture by a micrometer screw. The point at which the small beam of light thus isolated intersects the image plane is located by means of a filar micrometer. This gives a direct measure of the transverse aberration of that zone of the lens which contains the sliding hole, from which either the optical path difference or the ordinary longitudinal aberration of the zone can be readily computed. The aberration is then plotted graphically against the numerical aperture of the zone. The limitations and the precision of the method are discussed. The Foucault knife-edge test was found to be readily adaptable to the measurement of the chromatic aberration of a microscope objective. Typical curves for all these tests are given.
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