A method of determining the spherical aberration of a lens is given which requires relatively simple apparatus and which gives directly the retardation of different portions of the wave-front referred to any point along the axis andintermsof the wave length of the light employed. Data are given from which a convenient self-contained apparatus can be built to serve as an attachment on an optical bench of usual design. The method is based upon interference phenomena and the bench does not need to be provided with scale or micrometric measuring devices. With the attachment built as suggested, in addition to its dimensions, the measuring of the photographic plate gives all the data required and the one necessary exposure provides a permanent and complete record of the test.
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