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Note to page 15 There must be mentioned a third possibility for a cylindrical corrector n e a r t h e e q u i v a l e n t f o c u s. To correct the astigmatical cutting-length-difference k and the astigmatical focal- length-difference , this type of corrector must be a system of two air-spaced cylindrical lenses of short and nearly equal r.o.c. The first, a plano-concave cylinder equals the radii y_3m and y_3s of the distorted cone of light in the plane X ( vid. fig. ) and the second, a plano-convex-cylinder, located in the plane X, will bring the sagittal rays into the meridional focus. Each of the two lenses must be adjustable.

If the two lenses are made of the same material ( a light Boro-Silicate- Crown) a negligible amount of chromatical aberration will result. To avoid all chromatical residuals, the two components of the correc- tor can be constructed as an achromatical system. Both lenses are to be coated.

This device of a corrector being more complicated the author doesn't recommend it for ATM's. If wanted, the computation-formulae are dis- posable.

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