12. To understand the experiment above represented on the paper, we are first to consider the nature of the shadow D, v, green, as it is in appearance; that is, we are to consider what kind of light or lights can possibly come to this portion of the paper which we call the shadow D, v; and here it is plain, that this space, D, v, is illumined only by the white light * (I will call it) which comes from the small taper A, directly, and also by a small quantity of white light from B, not directly, but by reflection from the sides of the room, or from other objects. The direct red light coming from B, through the red glass c, is intercepted by D; and the small quantity of this red light, which can arrive at the space D, v, by reflection, is not worth mentioning; the green shadow D, v, therefore, is illumined by a small quantity of white light, and our business is to explain why it should appear green to the eye. *[I call it white light because it is nearly so, and because it answers all the purposes of perfectly white light in such an experiment, supposed to be made in a room without daylight. When actually compared with daylight, is is found to be yellowish, or even orange-coloured.]
