Patent GB-691530-A (GB)
Title: Improvements in or relating to optical systems
IPC: G02B17/08
Filed: 1949-10-12
Abstract: 691,530. Reflecting objectives. INTERNATIONAL POLAROID CORPORATION. Oct. 12, 1949 [Oct. 20, 1948; March 10, 1949; May 7, 1949], Nos. 26203/49, 26204/49 and 26205/49. Class 97 (i). An objective for use throughout a spectral range from within the ultra-violet region to within the infra-red region comprises a plurality of optically aligned catoptric and dioptric components having spherical surfaces and comprising a pair of mirror elements, the first having a central light-transmitting portion surrounded by a concave reflecting surface facing the short conjugate focus of the objective and the second having a convex reflecting sur- . face facing said concave reflecting surface, and one or more refractive elements transparent throughout the said spectral range and of such dimentions and form as to correct the mirror elements for spherical aberration and coma for differences in refraction caused by changes in refractive index of the order of 0.1 between the limits of said range while maintaining the position of the conjugate foci constant, and together with the mirror elements providing an objective of numerical aperture at least equal to 0.5, said second mirror element obscuring a small central portion of one refractive surface. Suitable materials for the dioptric components are calcium fluoride, lithium fluoride, fused quartz, and #-magnesium oxide. As shown in Fig. 3, the convex and concave reflecting elements are front-surface aluminium mirrors on elements VII, IX and correction of aberration is supplied by a thick plano-convex lens VI and a thick negative meniscus lens VIII, both of calcium fluoride, the latter serving also to prevent entry of dust into the objective through the aperture in the element IX. The objective is designed for use as an immersion objective in a microscope and the data given in the Specification allows for a quartz cover slide and water as the immersion liquid. In a modification the member VI is made in two parts, a planoconvex member of quartz and a contacting thick meniscus lens of calcium fluoride. These objectives may be used as " dry " objectives by replacing the plane surface by a curved surface having its centre of curvature at the short conjugate focus of the system. In a further arrangement the correcting elements are all placed on the short conjugate focus side of the convex mirror and consist of a doublet comprising an outer quartz double concave lens and a double convex lens of calcium fluoride. In a different embodiment (Fig. 4), the concave reflector is formed on the rear of a lens II and the convex reflector is formed on the front of a lens I, both of quartz. In a modification of this which may be used either as a " dry " objective or with a cover slide as an immersion objective, the convex mirror supporting lens and a piano convex lens having its plane face at the focus of the objective may both be of calcium fluoride. The Specification as open to inspection under Sect. 91, states without giving examples, that sodium chloride, potassium bromide, and potassium chloride may be used for the dioptric components and that the systems disclosed are adapted for the use of materials having a more restricted range of transmission, e.g. of glass which transmits visible light but not ultraviolet. This subject-matter does not appear in the Specification as accepted.