Corresponding points, &c.
“From Müller’s law,” Le Conte further says, “it is evident that each point—every rod or cone—in the retina has its invariable correspondent in the visual field, and vice versâ.”
|Law of visible direction.|
Le Conte’s law of visible direction states that, “Where the rays from any radiant strike the retina the impression is referred back along the ray line (the central ray of the pencil) into space, and therefore to its proper place.”
From these laws we understand why we see things in the relative positions which they occupy in space.
All the previous remarks are applicable to monocular vision.
C. Intensity.
Intensity.
A quotation from Helmholtz will best illustrate this point. He says, “If the artist is to imitate exactly the impression which the object produces on our eye, he ought to be able to dispose of brightness and darkness equal to that which nature offers. But of this there can be no idea. Let me give a case in point. Let there be in a picture-gallery a desert scene, in which a procession of Bedouins, shrouded in white, and of dark negroes, marches under the burning sunshine; close to it a bluish moonlight scene, where the moon is reflected in the water, and groups of trees, and human forms, are seen to be faintly indicated in the darkness. You know from experience that both pictures, if they are well done, can produce with surprising vividness the representation of their objects; and yet in both pictures the brightest parts are produced with the same white lead, which is but slightly altered by admixtures; while the darkest parts are produced with black. Both being hung on the same wall, share the same light, and the brightest as well as the darkest parts of the two scarcely differ as concerns the degree of their brightness.
How is it, however, with the actual degrees of brightness represented. The relation between the lightness of the sun’s light, and that of the moon, was measured by Wollaston, who compared their intensities with that of the light of candles of the same material. He thus found that the luminosity of the sun is 800,000 times that of the brightest light of a full moon.