Page:Mind (Old Series) Volume 12.djvu/362

 THE PERCEPTION OF SPACE, (ill.) 349 of these altering forms are infinite and continual. Out of the flux however, one phase always stands prominent. It is the form the object has when we see it easiest and best ; and that is when our eyes and the object both are in what may be called the ' normal ' position. In this position our head is upright and our optic axes either parallel or symmetri- cally convergent ; the plane of the object is perpendicular to the visual plane ; and if the object is one containing many lines it is turned so as to make them, as far as possible, either parallel or perpendicular to the visual plane. In this situa- tion it is that we compare all shapes with each other ; here every exact measurement and decision is made. 1 It is very easy to see why the normal situation should have this extraordinary pre-eminence. First, it is the position in which we easiest hold anything we are examining in our hands ; second, it is a turning-point between all right- and all left-hand perspective views of a given object ; third, it is the only position in which symmetrical figures seem symmetrical and equal angles seem equal ; fourth, it is often that starting-point of movements from which the eye is least troubled by axial rotations, by which tuperposttipn* of the retinal images of different lines and different parts of the same line is easiest produced, and consequently by which the eye can make the best comparative measurements in its sweeps. All these merits single the normal position out to be chosen. No other point of view offers so many aesthetic and practical advantages. Here we believe we see the object as it is ; elsewhere, only as it seems. Experience and custom soon teach us, however, that the seeming appearance passes into the real one by continuous gradations. They teach us, moreover, that seeming and being may be strangely inter- changed. Now a real circle may slide into a seeming ellipse ; now an ellipse may, by sliding in the same direction, become a seeming circle ; now a rectangular cross grows slant-legged ; now a slant-legged one grows rectangular. Almost any form in oblique vision may be thus a derivative of almost any other in ' primary ' vision ; and we must learn, when we get one of the former appearances, to translate it into the appropriate one of the latter class ; we must learn of what optical ' reality ' it is one of the optical signs. 1 The only exception seems to be when we expressly wish to abstract from particulars, and to judge of the general * effect'. Witness ladies trying on new dresses with their heads inclined and their eyes askance ; or painters in the same attitude judging of the 'values' in their pictures. 2 The importance of Superposition will appear later on,