It would be pedantic, perhaps, anywhere but in a treatise on aesthetics, to deny to this quality the name of expression; we might commonly say that the circle has one expression and the oval another. But what does the circle express except circularity, or the oval except the nature of the ellipse? Such expression _expresses_ nothing; it is really impression. There may be analogy between it and other impressions; we may admit that odours, colours, and sounds correspond, and may mutually suggest one another; but this analogy is a superadded charm felt by very sensitive natures, and does not constitute the original value of the sensations. The common emotional tinge is rather what enables them to suggest one another, and what makes them comparable. Their expression, such as it is, is therefore due to the accident that both feelings have a kindred quality; and this quality has its effectiveness for sense independently of the perception of its recurrence in a different sphere. We shall accordingly take care to reserve the term "expression" for the suggestion of some other and assignable object, from which the expressive thing borrows an interest; and we shall speak of the intrinsic quality of forms as their emotional tinge or specific value. _Physiology of the perception of form._ Sec. 20. The charm of a line evidently consists in the relation of its parts; in order to understand this interest in spatial relations, we must inquire how they are perceived.[4] If the eye had its sensitive surface, the retina, exposed directly to the light, we could never have a perception of form any more than in the nose or ear, which also perceive the object through media. When the perception is not through a medium, but direct, as in the case of the skin, we might get a notion of form, because each point of the object would excite a single point in the skin, and as the sensations in different parts of the skin differ in quality, a manifold of sense, in which discrimination of parts would be involved, could be presented to the mind. But when the perception is through a medium, a difficulty arises. Any point, a, in the object will send a ray to every point, _a', b', c',_ of the sensitive surface; every point of the retina will therefore be similarly affected, since each will receive rays from every part of the object. [Illustration of light rays]