That, then, being the sum of what I could tell you with certainty respecting the methods of sight, I have next to assure you that this faculty of sight, disciplined and pure, is the only proper faculty which the graphic artist is to use in his inquiries into nature. His office is to show her appearances; his duty is to know them. It is not his duty, though it may be sometimes for his convenience, while it is always at his peril, that he knows more;--knows the _causes_ of appearances, or the essence of the things that produce them. 124. Once again, therefore, I must limit my application of the word science with respect to art. I told you that I did not mean by 'science' such knowledge as that triangles on equal bases and between parallels are equal, but such knowledge as that the stars in Cassiopeia are in the form of a _W_. But, farther still, it is not to be considered as science, for an artist, that they are stars at all. What _he_ has to know is that they are luminous points which twinkle in a certain manner, and are pale yellow, or deep yellow, and may be quite deceptively imitated at a certain distance by brass-headed nails. This he ought to know, and to remember accurately, and his art knowledge--the science, that is to say--of which his art is to be the reflection, is the sum of knowledges of this sort; his memory of the look of the sun and moon at such and such times, through such and such clouds; his memory of the look of the mountains,--of the look of sea,--of the look of human faces.