When different wavelengths of light are employed for measuring the threshold, it is found, for example, that the eye is much more sensitive to blue-green light than to orange. The interesting feature of this kind of study is that the subject reports only that the light is light; he distinguishes no colour. If the intensity of a given wavelength of light is increased step by step above the threshold, a point comes when the subject states that it is coloured, and the difference between the threshold for light appreciation and this, the chromatic threshold, is called the photochromatic interval. This suggests that the rods give only achromatic, or colourless, vision, and that it is the cones that permit wavelength discrimination. The photochromatic interval for long wavelengths (red light) is about zero, which means that the intensity required to reach the sensation of light is the same as that to reach the sensation of colour. This is because the rods are so insensitive to red light; if the dark-adaptation curve is plotted for a red stimulus it is found that it follows the cone path, like that for foveal vision at all wavelengths.
A-horizontal-cross-section-of-the-human-eye-showing-theA horizontal cross section of the human eye, showing the major parts of the eye, including the …[Credits : Encyclopædia Britannica, Inc.]
Horizontal-section-of-the-eyeFigure 1: Horizontal section of the eye.[Credits : Encyclopædia Britannica, Inc.]
Structure-of-the-retinaStructure of the retina
Muscles-of-the-right-eyeMuscles of the right eye
Myopia-or-nearsightedness-can-be-corrected-with-glasses-that-haveMyopia, or nearsightedness, can be corrected with glasses that have concave lenses to allow near …[Credits : Encyclopædia Britannica, Inc.]
Hyperopia-or-farsightedness-can-be-corrected-with-glasses-that-containHyperopia, or farsightedness, can be corrected with glasses that contain convex lenses to reduce …[Credits : Encyclopædia Britannica, Inc.]
Visual-pathwaysVisual pathways
Perimeter-chart-showing-normal-visual-field-figures-on-the-perimeterFigure 2: Perimeter chart showing normal visual field; figures on the perimeter indicate degrees of …[Credits : Encyclopædia Britannica, Inc.]
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