THE FIELD oF VISIttx. The sensitiveness 01 the retina varies in different parts. The most sen sitive portion corresponds to the posterior pole of the eyeball, a region which is called on ac count of its color the yellow spot ur nmcaht taieft, and on amount of its special endowment the rellion of most distinct vision.; this area presents in its centre a depression called the fovea. The fovea has no nerve fibres, blood-vessels, or retinal elements excepting cones; even the rods are ab sent. The in-amuIu (and still more markedly the fovea) serves to receive all those luminous sen sations which give rise to distinct impressions of vision. When we wish to see anything distinctly, we turn the eyeballs toward the °Neel, so that the image will fall upon this spot. Other parts of the retina are much less sensitive to form and color, but not to variations in brightness.
Vision can. therefore, be divided into (1) cen tral or distinct, when the image of the object falls on the macula, and (2) peripheral. when the image falls on some other part of the retina.
The space before the eye within which objects are seen, even though indistinctly, constitutes the visual field; its dimensions for each eye are: Externally or toward the temple, 90°, above 50°, below 70°, and internally or toward the nose, 60°; these represent the limits for white light, the fields for colors being less extensive. Though not adapted for distinct vision or for colo• vision, the peripheral field is of the greatest use in giving its knowledge of the existence of objects placed outside of the direct line of vision; with out it our surroundings would have the appear ance which we get when we look through a tube. Though we see indistinctly with peripheral vision, eccentric portions of the retina are more sen sitive to differences of light, irrespective of form aml color, than the macula ; hence in a very dim light we can often see an object better when it is outside of the line of direct vision, that is, when we are not looking directly at it.