If the pupil is covered on one side, the side opposite to the image of diffusion disappears; that is to say, that part of the retinal image situated on the same side as the covered half of the pupil. This figure, then, is formed by rays which have not yet crossed the axis of the eye. If we place the luminous point at a distance to which the eye can accommodate itself, we see, through a moderate light, a small, round, luminous spot, without any irregularities. If the light, on the contrary, is intense, the image is radiated in every position of accommodation, and we merely find that on approaching nearer, the figure which was elongated, answering to a distant accommodation, gradually diminishes, grows rounder, and gives place to the vertically elongated figure, which belongs to the accommodation of a nearer point. When we examine a slender, luminous line, we behold images developed, which are easily foreseen, if for every point of the line we suppose radiating images of diffusion, which encroach on each other. The clearest portions of these images of diffusion mingle together and form distinct lines, which show multiplied images of the luminous line. Most persons will see two of these images; some, with the eyes in certain positions, will see five or six. [Illustration: Fig. 102.] To show clearly by experiment the connection existing between double images and radiated images from points, it is sufficient to make in a dark sheet of paper a small rectilinear slit, and at a little distance from one end, on a line with the slit, a small round hole, as shown at _a_ in fig. 102. Looking at it from a distance we shall see that the double images of the line have exactly the same distance between them that the most brilliant parts of the starred figure of diffusion have from the point, and that the latter are in a line with the first, as will be seen at _b_ (fig. 102), where in the image of diffusion of the luminous point, we only see the clearest parts of star _a_ of the figure. On lighted surfaces, to which the eye is not exactly accommodated, multiplied images are often remarked through the passage from light to darkness being made by two or three successive steps.