On the Sublime and Beautiful/Part IV/Chapter 23
ANOTHER principal property of beautiful objects is, that the line of their parts is continually varying its direction; but it varies it by a very insensible deviation; it never varies it so quickly as to surprise, or by the sharpness of its angle to cause any twitching or convulsion of the optic nerve. Nothing long continued in the same manner, nothing very suddenly varied, can be beautiful; because both are opposite to that agreeable relaxation which is the characteristic effect of beauty. It is thus in all the senses. A motion in a right line is that manner of moving, next to a very gentle descent, in which we meet the least resistance; yet it is not that manner of moving which, next to a descent, wearies us the least. Rest certainly tends to relax; yet there is a species of motion which relaxes more than rest; a gentle oscillatory motion, a rising and falling. Rocking sets children to sleep better than absolute rest; there is indeed scarce anything at that age which gives more pleasure than to be gently lifted up and down; the manner of playing which their nurses use with children, and the weighing and swinging used afterwards by themselves as a favourite amusement, evince this very sufficiently. Most people must have observed the sort of sense they have had on being swiftly drawn in an easy coach on a smooth turf, with gradual ascents and declivities. This will give a better idea of the beautiful, and point out its probable course better, than almost anything else. On the contrary, when one is hurried over a rough, rocky, broken road, the pain felt by these sudden inequalities shows why similar sights, feelings, and sounds are so contrary to beauty: and with regard to the feeling, it is exactly the same in its effect, or very nearly the same, whether, for instance, I move my hand along the surface of a body of a certain shape, or whether such a body is moved along my hand. But to bring this analogy of the senses home to the eye: if a body presented to that sense has such a waving surface, that the rays of light reflected from it are in a continual insensible deviation from the strongest to the weakest (which is always the case in a surface gradually unequal,) it must be exactly similar in its effects on the eye and touch; upon the one of which it operates directly, on the other, indirectly. And this body will be beautiful, if the lines which compose its surface are not continued, even so varied, in a manner that may weary or dissipate the attention. The variation itself must be continually varied.