228

though yellows and grays are sometimes visible. Afterglows follow volcanic eruptions of explosive character and are generally ascribed to the presence of minute dust particles in the air. The eruption of Krakatoa in 1883 was accompanied by most gorgeous afterglows which were observed throughout the world and persisted for several years. Similar effects were seen over a much smaller area after the outbursts of Mont Pelée and La Soufrière in May 1902. The name foreglow is given to such displays in the eastern sky before sunrise.

AFTER-IMAGE, AFTER-SENSATION and AFTER-PERCEPT are the terms used to denote the direct after-effects of the stimulation of a sense organ. These after-effects occur in almost all of the sense departments. A brief stimulation of the sense organ gives a primary sensation, then a pause of a fraction of a second, followed by a secondary sensation of the same quality as the primary sensation. After-images of touch follow after brief contact. They do not appear under ordinary circumstances, but may be observed if special conditions are produced; for example, a gentle tap of a point of a needle will be followed by a pause, then an after-sensation which differs from the primary sensation in that it seems to be produced from within the body, not from without. The effects of a temperature stimulus may persist for a time in the same quality as the primary sensation. After-taste and after-smell have been observed, but have not been studied. Auditory after-sensations, analogous to after-sensations of touch, are very weak and of brief duration.

After-images of vision are stronger and more permanent, consequently have been given much more attention by experimentalists. It has been found that after the retina has been stimulated by light for one second, or less, the primary image disappears quickly; an interval of less than two seconds is then followed by a positive after-image, that is, an after-image of the same quality as the primary image.

A stimulus of longer duration is followed immediately by the positive after-image, and this image may itself be followed by a negative after-image, that is, an image which differs very much in brightness from the primary image, or is of a different color. With some observers a brief stimulus is followed immediately by a negative after-image, which fades away quickly to be followed after an interval by a more permanent positive after-image. Several images may succeed each other immediately or be separated by an interval of time. A stimulus of still greater duration is followed directly by a negative after-image. In such cases the after-image is usually of a color that is complementary to the color of the primary image, especially if observed with closed eyes or if projected upon that is, seen while looking at - a gray background. The duration of the after-image varies with the intensity, duration and area of the stimulus. The results of experiments, under conditions such that the intensity of the light does not vary, have not as yet shown that any one color has more power to produce after-images than any other color. The greater the angular distance of the portion of the retina stimulated, from the fovea, the less distinct and the less durable is the afterimage. There seems to be no after-image at

an angular distance of 45 degrees or more from the fovea. The explanation for this fact may be physiological, or psychological, or both; that is, it may be due to the fact that the periphery of the retina is more easily fatigued than the fovea, or it may be due to lack of ability to attend to those portions of the retina which are not customarily attended to. When an object occupies the attention, the eye is so directed toward it that the image falls over the fovea; the mind does not ordinarily attend to images that are not over or very near the fovea.

A blow on the head may cause the afterimage to become less intense or to cease entirely. Electrical stimulation of the eye and optic nerve will change the character of the after-image and shorten the time of its duration. General fatigue will shorten the duration of the after-image; for example, it has been found that an after-image lasts about 30 per cent longer in the morning than in the evening. The distraction of attention in any manner has its effect on the course of the afterimage; when the attention is directed wholly upon the after-image the duration is one-third longer than when the attention is not concentrated upon it.

If one eye only be stimulated, an afterimage may appear in the unstimulated eye. Four hypotheses have been offered to explain this transfer of the image from one eye to the other: (1) The appearance is a phenomenon of binocular contrast. When one eye is stimulated by a bright colored light, and the other eye is stimulated by a very little gray light or is protected from all light, the contrasted color may be seen in the unstimulated eye during the time of stimulation, and this may leave an after-image in that eye. (2) A second hypothesis is that the eyes are accustomed to function together, and whatever affects one retina affects the other also. This may be considered as a modified form of the first hypothesis. (3) Another hypothesis is that the after-image has its seat in the centres in the brain, not in the end organ or retina, and that it may be seen in whichever eye is open. This hypothesis

seems to be overthrown by the fact that an electrical stimulation of the optic nerve produces a sensation like that produced by a flash of light, but no after-image follows. Another fact difficult for this hypothesis to explain is that if one eye be stimulated the after-image appears in the other eye only in case that eye be well darkened. (4) A fourth hypothesis is that the transfer of the after-image is not real but only apparent. In support of this hypothesis it has been found that when "that portion of the right eye which corresponds to the blind spot of the left eye was stimulated" (Franz), there was an apparent transfer of the image to the left eye; also if the unstimulated eye be disturbed or interfered with during the course of the after-image no change in the image may be observed, whereas if the stimulated eye be interfered with the image disappears. Consult Fechner. 'Elemente der Psycho-physik'; Franz, S. I., 'After-Images' (Monograph Supplement, Vol. III, No. 2, 1899, Psych. Rev.); Titchener, E. B., Ueber binocular Wirkungen monocular Reize.' See EYE; VISION.

AGABUS, ä'-ga-buz, a prophet_residing in Jerusalem in the time of Paul and said by tradition to be one of the 70 disciples of Jesus