Then, again, that very variety of nature, by its multiplicity, disposes the mind ever to look for a constant change and new effect, so that we are not easily startled by any actual unnaturalness, unless it be very strange indeed, and entirely out of harmony, one part with another, as we should be were one aspect only and constantly presented to us. This may be exemplified by a dark mirror--and, better still, by a Claude glass, as it is called, by which we look at nature through coloured glasses. We do not the less recognise nature--nay, it is impossible not to be charmed with the difference, and yet not for a moment question the truth. I am not here discussing the propriety of using such glasses--it may be right or it may be wrong, according to the purpose the painter may have; I only mean to assert, that nature will bear the changes and not offend any sense. The absolute naturalness, then, of the colour of nature, in its strictest and most limited sense, local and aerial, is not so necessary as that the eye cannot be gratified without it. And it follows, that agreeability of colour does not depend upon this strict naturalness. I said, that it is of the first importance that the colouring be agreeable _per se_; that, without any regard to a subject, the eye should be gratified by the general tone, the harmony of the parts, and the quality--namely, whether it be opaque or transparent, and to what degree. There are certain things that we greatly admire on this very account--such as all precious gems, polished and lustrous stones and marbles, especially those into which we can look as into a transparent depth. A picture, therefore, cannot be said to be well coloured unless this peculiar quality of agreeability be in it. To attain this, much exactness may be sacrificed with safety. It should be considered indispensable. And this perfect liberty of altering to a certain degree the naturalness of colouring, leads properly to that second essential--its adaptation to a subject, or its _expressing the sentiment_. For it is manifest, that if we can, without offending, alter the whole aspect of nature in most common scenes, we can still more surely do so when the scenes are at all ideal or out of the common character. And we can do it likewise without a sacrifice of truth, in the higher sense of _truth_, as a term of art or of poetry.