A study has been made of the resolving power of the human eye for Foucault test objects of different inherent contrast viewed through circular artificial pupil stops ranging from 0.30 of a millimeter to 7.0 millimeters in diameter. Two methods were used to make the resolution measurements. The first consisted of viewing the test objects through circular apertures in 10-mil blackened brass shim stock disks placed about 3 millimeters from the observer’s eye. Provision was made for illuminating the test objects both with tungsten lamps and with mercury arcs. The second method involved the use of test objects viewed in collimated light by means of a telescopic system for which the exit pupil could be varied from 0.40 of a millimeter to 2.0 millimeters. Both methods used equipment embodying the basic principles of the K.D.C. apparatus. The study included resolution measurements for rectangular and square Foucault test objects consisting of straight parallel black and white bands having an inherent contrast of approximately 94 percent. The coefficient of specific resolution was computed for test objects having lower values of inherent contrast. The resolution measurements, totaling over 100,000, were made by thirty-two different observers ranging from 18 to 76 years in age. The coefficient of specific resolution was found to be practically independent of age and observer for artificial pupil stops less than 0.75 of a millimeter in diameter. This suggests an effective consistency of the contrast threshold of the human retina as far as resolution measurements are concerned and indicates that the diffraction of light at the pupil stop is the primary factor controlling resolution. For artificial pupil stops of larger diameter, the coefficient of specific resolution was found to depend upon the individual observer but only with a maximum variation among the observers used of 30 percent. The composition of the test object was found to influence the coefficient of specific resolution to a certain extent with an indication that the best resolution was obtainable with test objects having the greatest number of elements.
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