. Animal and vegetable physiology considered with reference to natural theology. Natural theology; Physiology; Plant physiology; Biology. VISION. 407 from its original course to a particular side, we have only to turn the surface of the medium in such a manner as that the perpendicular line to that surface, contained within the denser medium, shall lie still farther on the same side. Thus, in Fig. 408, if we wish to turn the ray r s, from s o to s T, we must place the dense medium so that the perpendicular s p, which is within it, shall be still farther from s o, than s t is; that is, shall li
. Animal and vegetable physiology considered with reference to natural theology. Natural theology; Physiology; Plant physiology; Biology. VISION. 407 from its original course to a particular side, we have only to turn the surface of the medium in such a manner as that the perpendicular line to that surface, contained within the denser medium, shall lie still farther on the same side. Thus, in Fig. 408, if we wish to turn the ray r s, from s o to s T, we must place the dense medium so that the perpendicular s p, which is within it, shall be still farther from s o, than s t is; that is, shall lie on the other side of s r. The same rule applies to the contrary refraction of the ray s t from t v to t u, when it passes out of a dense, into a rare medium ; for the perpendicular t i must still be placed on the same side of t v as t u is situated. Let us now apply these principles to the case before us; that is, to the determination of the form to be given to a dense medium, in order to collect a pencil of rays, proceeding from a distant object. 409. accurately to a focus. We shall suppose the object in question to be very remote, so that the rays com- posing the pencil may be considered as being parallel to each other; for at great distances their actual deviation from strict parallelism is wholly insensible; and let a, b, c, d, e, (Fig. 409), repre- sent these rays. There must evidently be one of these rays (c), and only one, which, by continuing its rectilineal course, would arrive at the point (u). Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Roget, Peter Mark, 1779-1869. London : W. Pickering
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