HOW WE SEE IN THE LIGHT AND IN THE DARK

The retina of the eye contains a quantity of microscopic structures, some of which may be roughly described as rods, others as cones. The cones are scattered over the entire retina but are thickest at the very center. The rods have about the same distribution except that they are entirely lacking at the center. From pathological cases and from histology the function of these tiny structures has been determined.

A human eye which contains no rods has never been discovered but the eyes of fowls and pigeons show this defect, and these animals are apparently quite blind at night. All of their activities are carried on in the daylight. Hens do not even lay eggs in total darkness. Conversely the retinas of nocturnal animals, such as bats and owls and moles, are almost entirely lacking in cones but are richly supplied with rods. These and similar findings, together with the psychological differences of night and day vision, have led psychologists and biologists alike to conclude that the rods and cones, although they are very similar in structure, serve two very different functions. The cones provide us with daylight vision, apparently, while the rods, alone, are responsible for the manner in which we see in the dark. The rods are completely inactive in daylight but begin to respond to weak stimuli as soon as twilight (real or artificial) sets in. And the cones which are reacting continuously during every second that it is light, become functionally useless in the dark.

The cones alone are responsible for true colors. Many theories have been advanced to explain their functioning, but they are too long and too complicated to be reviewed here. This much can be stated, however; only three photo-chemical substances reacting to light produce all of our many different hues. The same substance reacting in different ways gives us both red and green, another both blue and yellow, and a third both black and white. When more than one substance responds to stimulation we get the intermediate colors which resemble one or more of these primary colors, for instance, purple which resembles both blue and red, and orange which is like both red and yellow. In normal eyes all three substances are found only at the center and consequently it is only at the center that all colors are visible. In partially color blind eyes the red-green substance is absent from the center as well as the periphery and the partially color blind, therefore, can see only the colors which contain either yellow or blue. In the absence of blue or yellow stimuli they see only grey. The totally color blind eye is completely deficient in cones. The totally color blind use their rods for daylight as well as dark vision.