This something which must always be present to enable us to see, is called Light; all objects are made visible to the sense of seeing by its agency.
Without light, natural or artificial, it would be impossible to distinguish one object from another.
Fig. 306.
That the Visual Rays pass from objects in straight lines to the eye may be proved by the following experiment (see [fig. 306]):—Pierce two screens with a large pin, and place them so that the holes are in a straight line with a flame, as the light of a candle or lamp. On fixing the eye to one of these holes we are able to see the flame; but if we slightly move the flame, one of the screens, or the eye, the flame is no longer visible. To be visible, the flame, the holes in the screens, and the eye must all be in the same straight line. See [fig. 306].
In [fig. 299] the picture plane is represented by the rectangle W X Y Z. Although the picture plane is here shown as a rectangle, it may be of any shape or of any size.
The observer is at S, looking through the picture plane at the cross R C O H. The observer is standing upon a horizontal surface, which is called the ground plane. If we are in a room, the window may be called a picture plane and the floor a ground plane.
The picture plane rests, as it were, upon the ground plane, in a line which passes from Y to Z. The two planes meet or intersect in this line, which is called the ground line. The ground line is sometimes called the picture line, or the measuring line.
The visual rays, by means of which the observer sees the cross, will, in their course from it to the eye, pass through the picture plane. These visual rays will intersect the picture plane in a number of points, and if we mark the true positions of these points the result will be a perspective image of the cross.
The rays are shown passing from the cross to the eye of the observer, and meeting the picture plane in points r, c, o, h; r being joined to o, and c to h, we have the perspective image of the cross as it would appear to the observer at S. Of course an infinite number of rays proceed from the cross to the eye of the observer; but it is quite evident that we need only consider those proceeding from the extremities of the object.