"In Fig. 50 the lines are all drawn so as to pass through a common point. With a little trouble one eye can be put into the position of this point--it is only necessary that the paper be held so that, with one eye shut, the other eye sees all the lines leaning neither to the right nor to the left. After a moment one can fancy the lines to be vertical staffs standing out of the plane of the paper.'... This illusion [says Mrs. Franklin] I take to be of purely mental origin. When a line lies anywhere in a plane passing through the apparent vertical meridian of one eye, and is looked at with that eye only... we have no very good means of knowing how it is directed in that plane.... Now of the lines in nature which lie anywhere within such a plane, by far the greater number are vertical lines. Hence we are peculiarly inclined to think that a line which we perceive to be in such a plane is a vertical line. But to see a whole lot of lines at once, all ready to throw their images upon the vertical meridian, is a thing that has hardly ever happened to us, except when they all have been vertical lines. Hence when that happens we have a still stronger tendency to think that what we see before us is a group of vertical lines." [Illustration: FIG. 50.] In other words, we see, as always, the most probable object. * * * * * The foregoing may serve as examples of the first type of illusions mentioned on page 86. I could cite of course many others, but it would be tedious to enumerate all the thaumatropes and zoetropes, dioramas, and juggler's tricks in which they are embodied. In the chapter on Sensation we saw that many illusions commonly ranged under this type are, physiologically considered, of another sort altogether, and that associative processes, strictly so called, have nothing to do with their production. _Illusions of the Second Type._ We may now turn to illusions of the second of the two types discriminated on page 86. In this type we perceive a wrong object because our mind is full of the thought of it at the time, and any sensation which is in the least degree connected with it touches off, as it were, a train already laid, and gives us a sense that the object is really before us. Here is a familiar example: "If a sportsman, while shooting woodcock in cover, sees a bird about the size and color of a woodcock get up and fly through the foliage, not having time to see more than that it is a bird of such a size and color, he immediately supplies by inference the other qualities of a woodcock, and is afterwards disgusted to find that he has shot a thrush. I have done so myself, and could hardly believe that the thrush was the bird I had fired at, so complete was my mental supplement to my visual perception."[100]