touch the square fj, because it is on the surface of the picture, but we cannot touch the square ghmb at the other end of the cube and can only measure it by the rules of perspective.
[ II]
The Point of Sight, the Horizon, and the Point of Distance
There are three things to be considered and understood before we can begin a perspective drawing. First, the position of the eye in front of the picture, which is called the Station-point, and of course is not in the picture itself, but its position is indicated by a point on the picture which is exactly opposite the eye of the spectator, and is called the Point of Sight, or Principal Point, or Centre of Vision, but we will keep to the first of these.
| Fig. 9. | Fig. 10. |
If our picture plane is a sheet of glass, and is so placed that we can see the landscape behind it or a sea-view, we shall find that the distant line of the horizon passes through that point of sight, and we therefore draw a line on our picture which exactly corresponds with it, and which we call the Horizontal-line or Horizon.[3] The height of the horizon then depends entirely upon the position of the eye of the spectator: if he rises, so does the horizon; if he stoops or descends to lower ground, so does the horizon follow his movements. You may sit in a boat on a calm sea, and the horizon will be as low down as you are, or you may go to the top of a high cliff, and still the horizon will be on the same level as your eye.
This is an important line for the draughtsman to consider, for the effect of his picture greatly depends upon the position of the horizon. If you wish to give height and dignity to a mountain or a building, the horizon should be low down, so that these things may appear to tower above you. If you wish to show a wide expanse of landscape, then you must survey it from a height. In a composition of figures, you select your horizon according to the subject, and with a view to help the grouping. Again, in portraits and decorative work to be placed high up, a low horizon is desirable, but I have already spoken of this subject in the chapter on the necessity of the study of perspective.