This section is from the "The elements of drawing & the elements of perspective" book, by J. M. Dent & Sons. Also see Amazon: The Elements Of Drawing & The Elements Of Perspective.
Is this a hard saying? It seems paradoxical, but it is by his recognition of the illusions and the veracities and the innocences of the eye, as it ministers to our faculty and perceptions in art, that Ruskin clears for us our working vision. Two more passages, developed on lines helpful to readers of the Elements, must yet be included: one on the drawing of a fir-tree, the other on the drawing of water One of the most marked distinctions between one artist and another, in the point of skill, will be found in their relative delicacy of perception of rounded surface; the full power of expressing the perspective, foreshortening, and various undulation of such surface is, perhaps, the last and most difficult attainment of the hand and eye. For instance: there is, perhaps, no tree which has baffled the landscape painter more than the common black spruce fir. It is rare that we see any representation of it other than caricature. It is conceived as if it grew in one plane, or as a section or a tree, with a set of boughs symmetrically dependent on opposite sides. It is thought formal, unmanageable, and ugly.
It would be so,power of the tree is not in that chandelier-like section. It is in the dark, flat, solid tables of leafage, which it holds out on its strong arms, curved slightly over them like shields, and spreading towards the extremity like a hand. It is vain to endeavour to paint the sharp, grassy, intricate leafage, until this ruling form has been secured; and in the boughs that approach the spectator, the foreshortening of it is like that of a wide hill country, ridge just rising over ridge in successive distances; and the finger-like extremities, foreshortened to absolute bluntness, require a delicacy in the rendering of them like that of the drawing of the hand of the Magdalene upon the vase in Mr. Rogers's Titian. Get but the back of that foliage, and you have the tree; but I cannot name the artist who has thoroughly felt it. Compare this fine relation of the Power of the Tree, with what is said of its Unity, in his section (in Letter III) on Radiation.
The other passage supplements the wonderful pages on Water and its illusions and depiction in Letter II:There is, perhaps, nothing which tells more in the drawing of water than decisive and swift execution; for, in a rapid touch the hand naturally falls into the very curve of projection which is the absolute truth; while in slow finish, all precision of curve and character is certain to be lost except under the hand of an unusually powerful master....
I believe it is a result of the experience of all artists, that it is the easiest thing in the world to give a certain degree of depth and transparency to water; but that it is next thing to impossible, to give a full impression of surface. If no reflection be given - a ripple being supposed - the water looks like lead: if reflection be given, it in nine cases out of ten, looks morbidly clear and deep.... Now, this difficulty arises from the very same circumstance which occasions the frequent failure in effect of the best drawn foregrounds, the change, namely of focus necessary in the eye in order to receive rays of light coming from different distances. Go to the edge of a pond, in a perfectly calm day, at some place where there is duckweed floating on the surface, - not thick, but a leaf here and there. Now, you may either see in the water the reflection of the sky, or you may see the duck-weed; but you cannot, by any effort, see both together.... Hence it appears, that whenever we see plain reflections of comparatively distant objects, in near water, we cannot possibly see the surface, and vice versa.... Hence, the ordinary effect of water is only to be rendered by giving the reflections of the margin clear and distinct (so clear they usually are in nature, that is impossible to tell where the water begins); but the moment we touch the reflection of distant objects, as of high trees or clouds, that instant we must become vague and uncertain in drawing, and, though vivid in colour and light as the object itself, quite indistinct in form and feature.