"Always," of course, is a wide word. In your copying series I have put a sketch by Turner in color from Nature; some few others of the kind exist, in the National Gallery and elsewhere. But, as a rule, from his boyhood to the last day of his life, he sketched only with the fine pencil point, and always the outline, more if he had time, but at least the outline, of every scene that interested him; and in general, outline so subtle and elaborate as to be inexhaustible in examination and uncopiable for delicacy.
Here is a sketch of an English park scene which represents the average character of a study from Nature by Turner; and here the sketch from Nature of Dumblane Abbey for the Liber Studiorum, which shows you what he took from Nature, when he had time only to get what was most precious to him.
21. The first thing, therefore, you have to learn in landscape, is to outline; and therefore we must now know precisely what an outline is, how it ought to be represented; and this it will be right to define in quite general terms applicable to all subjects.
We saw in the fifth Lecture[5] that every visible thing consisted of spaces of color, terminated either by sharp or gradated limits. Whenever they are sharp, the line of separation, followed by the point of your drawing instrument, is the proper outline of your subject, whether it represents the limits of flat spaces or of solid forms.
22. For instance, here is a drawing by Holbein of a lady in a dark dress, with bars of black velvet round her arm. Her form is seen everywhere defined against the light by a perfectly sharp linear limit which Holbein can accurately draw with his pen; the patches of velvet are also distinguished from the rest of her dress by a linear limit, which he follows with his pen just as decisively. Here, therefore, is your first great law. Wherever you see one space of color distinguished from another by a sharp limit, you are to draw that limit firmly; and that is your outline.
23. Also, observe that as your representing this limit by a dark line is a conventionalism, and just as much a conventionalism when the line is subtle as when it is thick, the great masters accept and declare that conventionalism with perfect frankness, and use bold and decisive outline, if any.
Also, observe, that though, when you are master of your art, you may modify your outline by making it dark in some parts, light in others, and even sometimes thick and sometimes slender, a scientifically accurate outline is perfectly equal throughout; and in your first practice I wish you to use always a pen with a blunt point, which will make no hair stroke under any conditions. So that using black ink and only one movement of the pen, not returning to thicken your line, you shall either have your line there, or not there; and that you may not be able to gradate or change it, in any way or degree whatsoever.
24. Now the first question respecting it is: what place is your thick line to have with respect to the limit which it represents—outside of it, or inside, or over it? Theoretically, it is to be over it; the true limit falling all the way along the center of your thick line. The contest of Apelles with Protogenes consisted in striking this true limit within each other's lines, more and more finely. And you may always consider your pen line as representing the first incision for sculpture, the true limit being the sharp center of the incision.
But, practically, when you are outlining a light object defined against a dark one, the line must go outside of it; and when a dark object against a light one, inside of it.
In this drawing of Holbein's, the hand being seen against the light, the outline goes inside the contour of the fingers.