SHADOW, in optics, a privation or di minution of light by the interposition of an opaque body ; or it is a plane, where the light is eithe r altogetherobstructed, or greatly weakened, by the interposition of some opaque body between it and the luminary. A shadow of itself is invisible ; and therefore, when we say we see a shadow, we partly mean that we see bo dies placed in the shadow, and illuminat ed by light reflected from collateral bo dies; and, partly, that we see the con fines of the light. If the opaque body that projects the shadow be perpendicu lar to the horizon, and the place it is pro jected on be horizontal, the shadow is called a right shadow ; and such are the shadows of men, trees, buildings, moun ta is, But if the opaque body be placed parallel to the horizon, the sha dow is called a versed shadow ; as the arms of a man stretched out. &c "The laws of the projection of sha dows from opaque bodies." 1. Every opaque tody projects a shadow in the same direction with its rays; that is, to wards the part opposite to the light. Hence, as either the luminary or the bo dy changes place, the shadow likewise changes. 2 Every opaque body projects as many shadows as there are luminaries to enlighten it. 3. As the light of-the luminary is more intense, the shadow is the deeper : hence the intensity of the shadow is measured by the degreesof light that space is deprived*of. 4. if a lumi nous sphere he equal to an opaque one it illum;nates, the shadow which this latter projects will be a cylinder, and conse quently will be propagated still equal to itself, to whatever distance the luminary is capable of acting ; so that if it be cut in any place, the plane of the section will be a circle, equal to a great circle of the opaque sphere. 5 If the luminous sphere be greater than the opaque one, the sha dow will be conical. if; therefore, the shadow be cut by a plane, parallel to the base, the plane of section will be a circle ; and that so much the less as it is a great er distance from the base. 6. If the lu
minous sphere be less than an opaque one, the shadow will be a truncated cone ; and, consequently, grows still wider and wider ; and, therefore, if cut by a plane parallel to the section, that plane will be a circle, so much the greater as it is fur ther from the base.
The sun, being vastly larger than the whole globe of the earth, must give all its shadows pointed, by reason that it il lumines more than half of them. In con sequence of this demonstration we might conclude, that all the sun's shadows must be less than the bodies that project them, and diminished more and more as they recede further and further. Now this would be true, were there any relation between the body illuminated and the body illumining ; but as all objects on the earth are so small in comparison of that star, the diminution of their shadows is imperceptible to the eye, which sees them always equal; i. e. either broader or narrower than the body that forms them : on this account, all the shadows caused by the sun are made in parallels. From the whole it appears that, to find the shadow of any body whatever opposed to the sun, a line must be drawn from the top of the luminary perpendicular to the place where the foot of the luminary is to be taken : and through this place an occult line is to be drawn through one of the angles of the plane of the object, and an other from the sun to the same angle; and the intersection of the twelines will show how far the shadow is to go : all the other lines must be drawn parallel here to. The shadows of the sun are equal in objects of the same height, though at a distance from each other. Experience teaches, that stiles, or elevations of the same height, removed to a distance from each other, do yet project equal shadows at the same time : for they are lengthen ing and shortening in proportion as the sun comes nearer, or recedes further off; one or other of which he is continually doing.