VISIBLE, something that is an object of sight or or something whereby the eye is affected, so as to produce a sensation.
The Cartesians say that light alone is the proper object of vision. But accord ing to Newton, colour alone is the proper object of sight ; colour being that pro perty of light by which the light itself is visible, and by which the images of opaque bodies are painted on the retina. Philosophers in general had formerly taken for granted, that the place to which the eye refers any visible object, seen by reflection or refraction, is that in which the visual ray meets a perpendicular from the object upon the reflecting or the refracting..plane. That this is the case with respect to plane mirrors is univer sally acknowledged ; and some experi ments with mirrors of other forms seem... to favour the same conclusion, and thus afford reason for extending the analogy to all cases of vision. If a right line be held perpendicularly over a convex or concave mirror, its image seems to make one with it. The same is the case with a right line held perpendicularly within water ; for the part_ which is with in the water seems to be a continuation of that which is without. But Dr. Bar row called in question this method of judging of the place of an object, and so opened a new field of inquiry and debate in this branch of science. This, with other optical investigations, he published in his Optical Lectures, first printed in 1674. According to him, we refer every point of an object to the place from which the pencils of light issue, or from which they would have issued, if no reflecting or refracting substance intervened. Pur suing this principle, Dr. Barrow proceed ed to investigate the place in which the rays issuing from each of the points of an object, and that reach the eye after one reflection or refraction, meet ; and he found that when the refracting surface was plane, and the refraction was made from a denser medium into a rarer, those rays would always meet in a place be tween the eye and a perpendicular to the point of incidence. If a convex mir ror be used, the case will be the same ; but if the mirror be plane, the rays will meet in the perpendicular, and beyond it, if it be concave. He also determined,
according to these principles, what form the image of a right line will take when it is presented in different manners to a spherical mirror, or when it is seen through a refracting medium.
M. Bouguer adopts Barrow's general maxim, in supposing that we refer objects to the place from which the pencils of rays seemingly converge at their entrance into the pupil. But when rays issue from below the surface of a vessel of wa ter, or any other refracting medium, lie finds that there are always two different places of this seeming convergence: one of them of the rays that issue from it in the same vertical circle, and therefore fall with different degrees of obliquity upon the surface of the refracting medium; and another of those that fall upon the surface with the same degree of obliquity, entering the eye laterally with respect to one another. He says, sometimes one of these images is attended to by the mind, and sometimes the other ; and different Images may be observed by different per sons. And he adds, that an object plunged in water affords an example of this duplicity of images.
From the principle above illustrated, several remarkable phenomena of vision may be accounted for : as—That if the distance between two visible objects be an angle that is insensible, the distant bodies will appear as if contiguous : whence, a continuous bodybeing the result of several contiguous ones, if the dis tances between several visibles attend insensible angles, they will appear one continuous body ; which gives a pretty illustration of the notion of a continuum. Hence also parallel lines, and long vistas, consisting of parallel rows of trees, seem to converge more and more, the further they are extended from the eye ; and the roofs and floors of long extended alleys seen, the foster to descend, and the lat ter to ascend, and approach, each other ; because the apparent magnitudes of their perpendicular intervals are perpetually diminishing, while at the same time we mistake their distance. See Priestley's Light and Colours.