A further illustration of the principle that shape-perception is a combination of active measurements and comparisons, and of remembrance and expectations, is found in a fact which has very great importance in all artistic dealings with shapes. I have spoken, for simplicity's, sake, as if the patches of colour on a blank (i.e. uninteresting) ground along which the glance sweeps, were invariably contiguous and continuous. But these colour patches, and the sensations they afford us, are just as often, discontinuous in the highest degree; and the lines constituting a shape may, as for instance in constellations, be entirely imaginary. The fact is that what we feel as a line is not an objective continuity of colour-or-light-patches, but the continuity of our glance's sweep which may either accompany this objective continuity or replace it. Indeed such imaginary lines thus established between isolated colour patches, are sometimes felt as more vividly existing than real ones, because the glance is not obliged to take stock of their parts, but can rush freely from extreme point to extreme point. Moreover not only half the effectiveness of design, but more than half the efficiency of practical life, is due to our establishing such imaginary lines. We are inevitably and perpetually dividing visual space (and something of the sort happens also with "musical space") by objectively non-existent lines answering to our own bodily orientation. Every course, every trajectory, is of this sort. And every drawing executed by an artist, every landscape, offered us by "Nature," is felt, because it is measured, with reference to a set of imaginary horizontals or perpendiculars. While, as I remember the late Mr G. F. Watts showing me, every curve which we look at is _felt as being_ part of an imaginary circle into which it could be prolonged. Our sum of measuring and comparing activities, and also our dramas of remembrance and expectation, are therefore multiplied by these imaginary lines, whether they connect, constellation-wise, a few isolated colour indications, or whether they are established as standards of reference (horizontals, verticals, etc.) for other really existing lines; or whether again they be thought of, like those circles, as _wholes_ of which objectively perceived series of colour patches might possibly be _parts._ In all these cases imaginary lines are _felt,_ as existing, inasmuch as we feel the movement by which we bring them into existence, and even feel that such a movement might be made by us when it is not. So far, however, I have dealt with these imaginary lines only as an additional proof that shape-perception is an establishment of two dimensional relationships, through our own activities, and an active remembering, foreseeing and combining thereof. CHAPTER VII FACILITY AND DIFFICULTY OF GRASPING