A bright object viewed directly by a normal human eye shows no perceptible coloourd fringes. From this it has been assumed by some that the eye is fairly well corrected chromatically, at least for the most luminous constituents of white light. On the other hand, if the same object be viewed through a filter transmitting only the extreme red and blue, it will appear with either a red or blue fringe, showing that for these extreme rays the eye is not corrected. Helmholtz passed monochromatic light through a small hole and found that when red light was used, the hole appeared in best focus when viewed from a distance of about 8 feet. With blue illumination it appeared brightest at about 1 1/2 feet, and with extreme violet but a few inches (nur einige Zolle). With these rough determinations of Helmholtz the question appears to have rested.
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P. G. Nutting (1914) studied this question.