The standard trial lens set also contains one red and one green filter lens (Fig. 2.5). Some trial lens also contains one yellow filter. These filter lenses are used to check over-correction and under-correction of refractive error. Under ordinary condition of illumination, the eyes are emmetropic for yellow, myopic for
green and hypermetropic for red. With each eye occluded in turn, the uncovered eye views the best line first through the red filter, which is rapidly exchanged for green filter successively. Figure 2.4:
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