Reflecting Microscopes

I HAVE read with particular interest the letter of E. M. Brumberg et al.1 mentioning the use of a reflecting microscope involving two spherical mirrors, since I have myself made three reflecting objectives: one of these was shown in process of figuring (N.A, 0.58) and one completed (N.A. 0.65) at the Physical Society's Exhibition in 1939. I used non-spherical mirror-pairs, Schwarzschild aplanats2,3, for a reason which may prove important if it is desired to exploit fully the possibilities of reflexion microscopy—I wished to reduce the fraction of the numerical aperture obstructed by the convex mirror.