[18] E. Mach: Analyse der Empfindungen, 2d ed., p. 98, Jena, 1900.
[19] R. Dodge: Amer. Jour. of Physiology, vol. 8, p. 317, 1903.
[20] E. B. Holt: Harvard Psych. Studies, Psych. Rev. Mon. Supplements, vol. 4, p. 42, 1903.
[21] S. Exner: Zeitschrift für Psych. u. Physiol., vol. 1, p. 46, 1890; E. Fick and A. Gürber: Berichte d. ophth. Gesellschaft in Heidelberg, 1889; E. B. Holt: op. cit. p. 4.
[22] Purkinje, 1825; reprinted in Aubert's Physiol. Stud. über d. Orientirung, p. 117, Tübingen, 1888.
[23] Image no. 5 appeared with the concavity in front. In the centre of the figure appeared a dark grayish splotch of light, very much darker than the rest of the image. This is due, most probably, to the presence of Charpentier's phenomenon of recurrent bands. If this happens in this figure the ends of the recurrent bright images would overlap while the centres would not, so that the black bands appearing, as it were, through interstices in the central part of the figure, would seem like a dark splotch, especially since the outlines of the bands are vague and hazy. The back end of the figure had the effect of being vertical or nearly so. This is probably due to the same cause as that which made the circular figure to appear as it did, namely, the negative after-image overlapping the positive after-image. The front of this black image is usually of about the same shape as the front of the real figure which it follows. If this is so, then, in this case, it would make the back part of the image pretty nearly vertical.
[24] The intensity of the objective stimulation may be represented by the line AB. If there were no reënforcement of stimulation the whole figure would be flat on top and of this height. The difference between AB and AC, or BC, represents the increase of intensity due to irradiation at the most favorably situated portion of the figure. The other portions receive increments proportional to their location, as indicated in the diagram.
[25] Favorable localization will of course depend largely on the shape of the figure in which the point is situated. Thus one in the angle of a triangle or at the horn of a crescent would have much less reënforcement of excitation than another point, say halfway down the side.
[26] Bidwell describes this "black process" or "negative after-image" of a bright, white light as being of a blackness more intense than the ordinary blackness of an entirely dark room. This is perfectly true. The black image, however, lasts for a very much longer time than the recurrent images of the same light. Often this velvety black band would trail along behind the moving light for the distance of a foot or more, gradually lightening into the darkness of the background.
[27] The intensities used with white light are all less than an eight-candle-power electric lamp placed at about a foot behind the opening and covered with two pieces of ground glass.