To set this matter in somewhat a clearer light, the familiar instance of a common jack, with a circular fly, may serve our purpose better than a more elegant form: preparatory to which, let the figure [Fig. 14 T p. I] be consider'd, which represents the eye, at a common reading distance viewing a row of letters, but fix'd with most attention to the middle letter A. [Illustration: Fig. 14] Now as we read, a ray may be supposed to be drawn from the center of the eye to that letter it looks at first, and to move successively with it from letter to letter, the whole length of the line: but if the eye stops at any particular letter, A, to observe it more than the rest, these other letters will grow more and more imperfect to the sight, the farther they are situated on either side of A, as is express'd in the figure: and when we endeavour to see all the letters in a line equally perfect at one view, as it were, this imaginary ray must course it to and fro with great celerity. Thus though the eye, strictly speaking, can only pay due attention to these letters in succession, yet the amazing ease and swiftness, with which it performs this task, enables us to see considerable spaces with sufficient satisfaction at one sudden view. Hence, we shall always suppose some such principal ray moving along with the eye, and tracing out the parts of every form, we mean to examine in the most perfect manner: and when we would follow with exactness the course any body takes, that is in motion, this ray is always to be supposed to move with the body. In this manner of attending to forms, they will be found, whether _at rest_, or _in motion_, to give _movement_ to this imaginary ray; or, more properly speaking, to the eye itself, affecting it _thereby_ more or less _pleasingly_, according to their different _shapes_ and _motions_. Thus, for example, in the instance of the jack, whether the eye (with this imaginary ray) moves slowly down the line, to which the weight is fix'd, or attends to the slow motion of the weight itself, the mind is equally fatigu'd: and whether it swiftly courses round the circular rim of the flyer, when the jack stands; or nimbly follows one point in its circularity whilst it is whirling about, we are almost equally made giddy by it. But our sensation differs much from either of these unpleasant ones, when we observe the curling worm, into which the worm-wheel is fixt [Fig. 15 L p. I]: for this is always pleasing, either at rest or in motion, and whether that motion is slow or quick. [Illustration: Fig. 15] That it is accounted so, when it is _at rest_, appears by the ribbon, twisted round a stick (represented on one side of this figure) which has been a long-establish'd ornament in the carvings of frames, chimney-pieces, and door-cases; and call'd by the carvers, _the stick and ribbon ornament_: and when the stick, through the middle is omitted, it is call'd the _ribbon edge_; both to be seen in almost every house of fashion.