The two horizontal lines are equal in measurement; they are unequal to the eye. Why? One suggestion is that _the eye moves freely along the one, and hesitatingly and obstructedly along the other_; the obliques tempt out, in the one case, and hem in, in the other. The suggestion can be tested; for movements of the eyes can be recorded; and it turns out to be correct. The eyes, in passing over a line, like the lines of the figure or of a printed page, move by sweeps or jerks; they go so far, halt, and start again. Experiment shows that movements along the lower horizontal take a longer sweep, and oftentimes come to a halt only when they have shot beyond the end-points of the line; whereas movements along the upper horizontal are themselves shorter, and frequently come to a halt before the extremities of the line have been reached. Here, then, is a kinæsthetic context to carry the meanings ‘longer’ and ‘shorter.’ Is the analysis adequate? Not for every case; the illusion is found to vary with _our general attitude toward the figures_. If we take them as wholes, the large open area below and the closed diamond-shaped area above strike the attention; we say, from total impression, that the lower horizontal is the longer. If, however, we take the figures critically, part by part, limiting our attention to the horizontals and disregarding the obliques, then the illusion is greatly reduced and may, with practice, disappear. Here, then, is a second context, which involves a brain-habit. Another suggestion is that _linear perspective_ may be at work; the larger figure is a book opening toward you, the smaller is a book opened away from you; the lower horizontal is therefore further off, and should (if the two books were of the same size) be smaller than the upper; since it is not, the lower book is seen as the larger. There are, without doubt, many figures in which perspective influences the perception; but there seems to be no reason to invoke it here. A fourth suggestion is that we read into the figures _ideas of our own muscular state_; the lower figure has room to expand, it is stretching and yawning; the upper is cramped and huddled; and so the illusion of length is produced. There is no doubt, again, that this putting of oneself in place of the lines plays a part in certain perceptions; but its influence here is negatived by the swallow figure; the birds flying toward each other are further apart than those flying from each other. On the whole, we may be satisfied with the two contexts first mentioned; the discussion shows, however, how many and how various motives may enter in to determine an illusory perception. [Illustration]