. The Composition of sea-water : comparative and descriptive oceanography. Seawater -- Composition. SECT. 3] APPLICATIONS OF THE GYROrENDULUM 333 observe in full daylight objects generally brighter than magnitude zero with astronomical telescopes having apertures as small as 30 mm and to reach first magnitude objects with apertures of about 50 mm. The brighter stars and planets (listed in Table I) can often be seen with telescopic aid in daylight because the light of a point source or star can be con- centrated optically by the ratio of the area of the entrance and exit pupils of the telescope
. The Composition of sea-water : comparative and descriptive oceanography. Seawater -- Composition. SECT. 3] APPLICATIONS OF THE GYROrENDULUM 333 observe in full daylight objects generally brighter than magnitude zero with astronomical telescopes having apertures as small as 30 mm and to reach first magnitude objects with apertures of about 50 mm. The brighter stars and planets (listed in Table I) can often be seen with telescopic aid in daylight because the light of a point source or star can be con- centrated optically by the ratio of the area of the entrance and exit pupils of the telescope (when exit pupil equals the pupilary diameter of the eye), while the brightness of an extended source, such as the background sky, is not in- creased by optical means. Optical systems can, however, reduce the intensity of background illumination to values considerably below the naked-eye level. Successful observation of stars in daylight involves physiological as well as physical effects. Since the threshold of sensitivity of the human eye decreases as the intensity of total stimulation rises, there are some advantages to be 2 MILE RADIUS. 1 MILE RADIUS Fig. 8. Difference between 37 Geon and Loran fixes taken simultaneously from Chain at sea. realized by placing the telescope in shaded observing conditions. It is also found that elimination of the polarized component of scattered light can heighten the apparent contrast of star images observed 90° or so from the sun. Polaroid niters dim the stars nearly a full magnitude but under favorable conditions, as when the air is free of large particles, they may enhance contrast by reducing the background brightness of the sky an even greater amount. The apparent sharpness and contrast of a foveal image can be enhanced by stimulating the surrounding portions of the retina—the phenomenon of spatial induction. But if the extra-foveal illumination is too bright the contrast of the foveal image is reduced. Thus a variable level of
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