During the past three years several infra-red microspectrometers have been described. The factors affecting the design of reflecting objectives suitable for this purpose are discussed and it is shown that for use in the rock-salt region of the infra-red it is possible to use a bi-spherical objective of 0.8 numerical aperture with a central obstruction of 40%. The dimensions of three satisfactory objectives are given.
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K. P. Norris (1954) studied this question.
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