IOG SCIENTIFIC RECREATIONS. 



which M. Helmholtz represents thus (fig. 101) : at c as it is presented to the 

 right eye, and at d as seen by the left. 



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 h'alf 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 approach- 

 ing nearer, the figure which was elongated, answering to a distant accommo- 

 dation, 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 eacK 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. 



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. 



A series of facts which have been collected under the title of irradiation, 

 and which show that brightly-lighted surfaces appear larger than they are 

 in reality, and that the dark surfaces which surround them appear diminished 

 to a corresponding degree, explains this by the circumstance that the lumi- 

 nous sensation is not proportional to the intensity of the objective light. 

 These phenomena affect very various appearances, according to the form of 

 respective figures ; they are generally seen with the greatest ease and inten- 

 sity when the eye is not exactly accommodated to the object examined, 

 either by the eye being too near or too far off, or by using a concave or con- 

 vex lens, which prevents the object being seen clearly. Irradiation is not 

 completely wanting, even when the accommodation is exact, and we notice 

 it clearly in very luminous objects, above all when they are small ; small 



