2. Irradiation β. As the apparent form of the moving image becomes distinctly perceptible, such irradiation takes place within the confines of the stimulated portion of the retina, so as to make the excitation present at favorably situated localities more intense than that of other places. The portions which are so situated as to receive this reënforcement are the first to enter consciousness. The various phenomena discussed earlier in this paper furnish examples of this process, as well as the phenomenon of the curved image. 3. Irradiation γ. After, and in part during, the rise and development of the reënforcing irradiation, emanations of nervous excitation of small intensity proceed from the borders of the stimulated portions and from the after-images, rapidly extending themselves over the retina and gradually decreasing in intensity. 4. Irradiation δ. When two fields of different intensities are brought into juxtaposition, the field having the greater intensity will enlarge itself at the expense of the other. This constitutes what has been usually termed irradiation, and is observable with stationary objects. This enlargement varies with the time during which it is observed, the absolute intensity of the light employed, and the relative differences in intensity of the two fields. Its angular extent under determinable conditions is constant, although it varies considerably from one observer to another, and with the same observer at different times. Its physiological explanation is probably similar to that of the other kinds of irradiation, viz., the spreading of nervous excitation over or through the layers of the retina, although various factors of accommodation, dispersion, achromatism, astigmatism, etc., enter in and modify the totality of the phenomenon. It will be noticed that reënforcement occurs in this kind of irradiation as well as in certain of the other forms. The sides of the dark fields upon which this form of irradiation shows itself appear curved inward at the centre, apparently showing the presence of a greater excitation in the lighter fields next to the centre of the darker ones. 5. Irradiation ε. When a luminous object has been observed for a long time (from thirty seconds to several minutes), the whole surrounding field will be flooded by a faint haze of light, which within certain limits increases in intensity the longer the stimulation is present. This phase has many characteristics of the first and most rapid kind of irradiation, and possibly represents a discontinuance of functioning, through fatigue of certain nervous mechanisms which prevent the spreading, or inhibit the perception, of this irradiatory excitation after the form of the object is distinguished clearly. It is probably largely due to such a mechanism that we are enabled to perceive as clearly and sharply as we do the outlines of objects which differ greatly in intensity from their backgrounds.