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.
You will probably find, also, that some parts of the stone, or of the paper it lies on, look luminous through the opening; so that the little circle then tells as a light spot instead of a dark spot. When this is so, you cannot imitate it, for you have no means of getting light brighter than white paper: but by holding the paper more sloped towards the light, you will find that many parts of the stone, which before looked light through the hole, then look dark through it; and if you can place the paper in such a position that every part of the stone looks slightly dark, the little hole will tell always as a spot of shade, and if your drawing is put in the same light, you can imitate or match every gradation. You will be amazed to find, under these circumstances, how slight the differences of tint are, by which, through infinite delicacy of gradation, Nature can express form.
If any part of your subject will obstinately show itself as a light through the hole, that part you need not hope to imitate. Leave it white; you can do no more.
When you have done the best you can to get the general form, proceed to finish, by imitating the texture and all the cracks and stains of the stone as closely as you can; and note, in doing this, that cracks or fissures of any kind, whether between stones in walls, or in the grain of timber or rocks, or in any of the thousand other conditions they present, are never expressible by single black lines, or lines of simple shadow. A crack must always have its .
The figure a, Fig. 5., is very dark, but this is to give an example of all kinds of depths of tint, without repeated figures complete system of light and shade, however small its scale. It is in reality a little ravine, with a dark or shady side, and light or sunny side, and, usually, shadow in the bottom. This is one of the instances in which it may be as well to understand the reason of the appearance; it is not often so in drawing, for the aspects of things are so subtle and confused that they cannot in general be explained; and in the endeavour to explain some, we are sure to lose sight of others, while the natural overestimate of the importance of those on which the attention is fixed causes us to exaggerate them, so that merely scientific draughtsmen caricature a third part of Nature, and miss two thirds. The best scholar is he whose eye is so keen as to see at once how the thing looks, and who need not therefore trouble himself with any reasons why it looks so: but few people have this acuteness of perception; and to those who are destitute of it, a little pointing out of rule and reason will be a help, especially when a master is not near them. I never allow my own pupils to ask the reason of anything, because, as I watch their work, I can always show them how the thing Is, and what appearance they are missing in it; but when a master is not by to direct the sight, science may, here and there, be allowed to do so in his stead.
Generally, then, every solid illumined object - for instance, the stone you are drawing - has a light side turned towards the light, a dark side turned away from the light, and a shadow, which is cast on something else (as by the stone on the paper it is set upon). You may sometimes be placed so as to see only the light side and shadow, sometimes only the dark side and shadow, and sometimes both or either without the shadow; but in most positions solid objects will show all the three, as the stone does here.
Hold up your hand with the edge of it towards you, as you sit now with your side to the window, so that the flat of your hand is turned to the window.You will see one side of your hand distinctly lighted, the other distinctly in shade. Here are light side and dark side, with no seen shadow; the shadow being detached, perhaps on the table, perhaps on the other side of the room; you need not look for it at present.
Take a sheet of note-paper, and holding it edgeways, as you hold your hand, wave it up and down past the side of your hand which is turned from the light, the paper being of course farther from the window. You will see, as it passes, a strong gleam of light strike on your hand, and light it considerably on its dark side. This light is reflected light. It is thrown back from the paper (on which it strikes first in coming from the window) to the surface of your hand, just as a ball would be if somebody threw it through the window at the wall and you caught it at the rebound.
Next, instead of the note-paper, take a red book, or a piece of scarlet cloth. You will see that the gleam of light falling on your hand, as you wave the book, is now reddened. Take a blue book, and you will find the gleam is blue. Thus every object will cast some of its own colour back in the light that it reflects.
Now it is not only these books or papers that reflect light to your hand: every object in the room on that side of it reflects some, but more feebly, and the colours mixing all together form a neutral l light, which lets the colour of your hand itself be more distinctly seen than that of any object which reflects light to it; but if there were no reflected light, that side of your hand would look as black as a coal.