As in the perception of two points of contact with the skin, so we find the retina ceases to be able to distinguish the difference between two luminous points, if they be brought to a focus at a distance of less than.002 mm. from one another. This distance nearly corresponds to the diameter of the cones, and it is supposed that the rays from two luminous points must come upon two different cones in order to be visible as two distinct objects. The cones are, however, very irregularly distributed over the retina, being packed closely together at the yellow spot, and scattered more and more widely apart as one passes to the peripheral parts of the retina. It is only at the yellow spot that the cones, which are here very thin, are so close together as.002 mm., so that this estimation of the size of visual areas could only hold good of the yellow spot, and toward the peripheral parts the power of discrimination must be much less keen. This is found to be the case, for in ordinary vision everything seen clearly with a sharp outline must be brought upon the yellow spot. This is spoken of as "direct vision." The images falling on the other parts of the retina are but dim and indistinct outlines, and these are spoken of as "indirect vision".
The stimulus need only be applied for a very short time to cause a distinct sensation, for we can readily see a single electric spark; and it need only affect an extremely small part of the retina, as a minute speck of light can be seen by direct vision, and a very feeble ray suffices to stimulate the retina. The amount of stimulation produced depends upon (1) the intensity of the light, i. e., the amount of light received in a given area; (2) the duration of its application; (3) the extent of retina to which it is applied; (4) the part of the retina stimulated; (5) the darker the background the weaker the illumination we can distinguish, i. e., the greater the stimulating effect of a weak light; (6) by fatigue the retina loses its power of appreciating light, and more stimulus is required to produce a given effect. On waking, the daylight is at first dazzling, but soon the retina can bear the stimulus. An increase of intensity does not cause an exactly proportional increase of stimulation, for we find the more the light is intensified the less we notice a fresh increment of light until a degree of intensity is arrived at, when no further addition can be detected, and the light becomes blinding. The less the absolute intensity of two lights the better we distinguish any difference that may exist between them.
Fig. 233. Section of the retina at the yellow spot, showing the great number of cones (a) at this point, and the thinness of the other layers. (Cadiat).
The effect lasts for an appreciable time after the stimulus has been removed, particularly if the light be very intense. This can be observed when a brilliant point is in rapid motion; instead of a point a streak of light is seen. Thus, part at least of the trail of falling stars is caused by the persistence of the stimulation, and a luminous body rapidly rotated gives the impression of a circle of fire.
When the stimulus is very intense, such as an electric light, or when we look at a bright object like the globe of a lamp steadily for some time, the effect persists, and after the eyes are shut we see a faint image of the object. This is called the positive after image. If the retina be exposed to a bright light until it be fatigued, and then suddenly turning we gaze at a white wall, the bright part of the positive after image is replaced by a dark figure which is termed the negative after image.
A strong stimulus applied to the retina spreads from the part upon which the bright image falls to those in its immediate neighborhood, so that the bright object looks larger. This phenomenon is called irradiation. It helps to explain many of the peculiarities of vision.