A convenient distinction is made between hallucinations and illusions. Hallucinations are defined as appearances wholly due to fancy; illusions, as fanciful perceptions of objects actually seen. There is also a hybrid case which depends on fanciful visions fancifully perceived. The problems we have to consider are, on the one hand, those connected with "_induced_" vision, and, on the other hand, those connected with the interpretation of vision, whether the vision be _direct_ or _induced_. It is probable that much of what passes for hallucination proper belongs in reality to the hybrid case, being an illusive interpretation of some induced visual cloud or blur. I spoke of the ever-varying patterns in the optical field; these, under some slight functional change, may become more consciously present, and be interpreted into fantasmal appearances. Many cases could be adduced to support this view. I will begin with illusions. What is the process by which they are established? There is no simpler way of understanding it than by trying, as children often do, to see "faces in the fire," and to carefully watch the way in which they are first caught. Let us call to mind at the same time the experience of past illnesses, when the listless gaze wandered over the patterns on the wall-paper and the shadows of the bed-curtains, and slowly evoked the appearances of faces and figures that were not easily laid again. The process of making the faces is so rapid in health that it is difficult to analyse it without the recollection of what took place more slowly when we were weakened by illness. The first essential element in their construction is, I believe, the smallness of the area covered by the glance at any instant, so that the eye has to travel over a long track before it has visited every part of the object towards which the attention is directed generally. It is as with a plough, that must travel many miles before the whole of a small field can be tilled, but with this important difference--the plough travels methodically up and down in parallel furrows; the eye wanders in devious curves, with abrupt bends, and the direction of its course at any instant depends on four causes: (1) on the easiest sequence of muscular motion, speaking in a general sense, (2) on idiosyncrasy, (3) on the mood, and (4) on the associations current at the moment. The effect of idiosyncrasy ft excellently illustrated by the "Number-Forms," where we observe that a very special sharply-defined track of mental vision is preferred by each individual who sees them. The influence of the mood of the moment is shown in the curves that are felt appropriate to the various emotions, as the lank drooping lines of grief, which make the weeping willow so fit an emblem of it. In constructing fire-faces it seems to me that the eye in its wanderings tends to follow a favourite course, and it especially dwells upon the marks that happen to coincide with that course. It