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96 KAPBELIAN July
outer zones which contribute most of the aberration in any system form the image in these systems and must therefore be highly corrected. Because of the location of the image between the corrector and mirror, many of these objectives do not lend themselves to ready utilization as taking lenses caused by shutter problems, nor are they adapted for use in ordinary projection systems. IMMERSION OBJECTIVES
The brightness of an image depends upon the ratio of indexes of refraction in the image and object spaces as well as upon the angle of the extreme ray (equation (1)). Thus the use of an immersion fluid of index 1.5 between the focal plane and the rear element of an immersion objective more than doubles the "speed" of the lens. It is thereby possible to obtain relative apertures of //0.4 or slightly faster with the accompanying advantages or disadvantages of increased theoretical resolving power and reduced depth of focus.
The difficulties and inconveniences associated with the immersion principle limit its use to very few applications. Bracey54 employed an enlarged oil-immersion microscope objective of //0.36 aperture for astronomical photography. One form of the Djian30 objective not dissimilar to that shown in Fig. 3b, is especially adapted for immersion use and attains an aperture of //0.54 through oiling the film to the rear plane surface of the system. A variety of high-speed Schmidts, including solid modifications, have been described by Hendrix and Christie55 and an aperture of //0.3 has been attained by Baker56 utilizing a solid system. Nicoll57 has shown an immersion Schmidt system of large aperture applied to picture taking and to television projection. Solid Schmidts and solid concentric systems of approximately//! aperture or greater are to be found in Henyey and Greenstein.47 In those applications where the ultimate in light-gathering power is essential and the inconveniences of the immersion systems are secondary considerations, these systems represent the only purely optical means whereby this extreme lens speed may be attained.
APPLICATIONS
There are a number of reasons to account for the fact that very few extreme-aperture objectives are available commercially: they require extensive design time, are costly in view of the complexity of construction and the limited production for a specialized market, and the performance at present of all-refracting objectives having spherical surfaces only is at best mediocre in comparison with well-designed objectives of //1. 5 or//2 aperture. An//0.85 objective may resolve 15