File:A manual of elementary geometrical drawing, involving three dimensions. Designed for use in high schools, academies, engineering schools, etc. - and for the self-instruction of inventors,artizans, (14579365567).jpg

Identifier: manualofelementa01warr (find matches)
Title: A manual of elementary geometrical drawing, involving three dimensions. Designed for use in high schools, academies, engineering schools, etc. : and for the self-instruction of inventors,artizans, etc. ... In five divisions ..
Year: 1867 (1860s)
Authors: Warren, S. Edward (Samuel Edward), 1831-1909
Subjects: Geometrical drawing Mechanical drawing
Publisher: New York : J. Wiley and son
Contributing Library: The Library of Congress
Digitizing Sponsor: The Library of Congress

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make equal angles with a perpendicular to a sur-face. But wC is the incident and Ce the reflected ray, to the eye ;hence at d is the point, on the surface, where they make equalangles with the perpendicular (normal) c?C, and the vertical line atd—d is the lightest line. 187. MemarJc.—The qvxestion may here arise, If all the hghtthat is reflected towards the eye is reflected from d—as it appearsto be—how can any other point of the body be seen ? To answerthis question requires a notice—;^?5^, of the difference betweenpolished and dull surfaces; and, second, between the case of lightcoming wholly va. one direction, ov princijoally in one direction. Ifthe cylinder CC, considered as perfectly polished, were deprived ofall reflected light from the air and surrounding objects, the fine atd—d would reflect to the eye all the light that the body wouldremit towards the eye, and would appear as a line of brilliant light,while other parts remitting no light whatever would be totally
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n J- - -^ 1 ^^V-. /,/-wV„-^ The >tii,t.t Kiun, EiiillTi .HiilLTo 4M l„,ta;. K SHADING. 73 invisible. Let us now suppose a reflecting medium, though animperfect one, as the atmosphere, to be thrown around the body.By reflection, every part of the body vtould receive some hght fromall directions, and so Avould remit some light to the eye, making thebody visible, though famtly so. But no body has a polish that isabsolutely perfect; rather, the great majority of those met with inengineering art have entirely dull surfaces. Now a dull surface,greatly magnified, may be supposed to have a structure like thatshown in PI. IX., Fig. 97, in which many of the asperities may besupposed to have one little facet each, so situated as to remit to theeye a ray received, by the body directly from the principal sourceof light. 188. Having thus proved that a cylinder placed before our eyescan be seen, we may proceed with an explanation of the distribu-tion of tints, h is midway between d and t. At

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