The following definitions of the prin cipal features in the science and applica tion of perspective will prove useful to the student, viz. projection delineates objects in piano, by means of right lines called rays, supposed to be drawn from every angle of the object to particular points. When the objects are angular, these rays necessarily form pyramids, having the plane or superficies, whence they proceed, for their basis ; but when drawn from, or to, circular objects, they form a cone.
Ichnography, or ichnographic projec tion, is described by right lines parallel among themselves, and perpendicular to the horizon, from every angle of every object, on a plane parallel to the hori zon. The points where the perpendicu lar lines or rays cut that plane being joined by right lines. The figure pro jected on the horizontal plane is likewise called the plan or seat of that object on the ground plane. The points are the scites, or seats, of the angles of the ob ject. The lines are the seats of the sides. By- this we are to understand how the basis of figures represented as super structures stand, or are supported ; and we are further enabled to judge of, in deed to measure, their several parts, and their areas.
Orthography represents the vertical position and appearance of an object ; hence orthographic projection is called the elevation. When we thus see front of a house, we give it that term; but when the side is displayed, we call it the profile. If we suppose a house, or other object, to be divided by a plane passing perpendicularly through it in a line at right angles with the point, we call it the lateral section ; but if the plane pass in a direction parallel with the front, it is termed a longitudinal section. If the plane passes in neither of the former directions (not however deviating from the vertical) it is said to be an ob lique section.
These give us the modes of laying down plans, of shewing the parts, and the manner in which the interiors of edifices are arranged; consequently are indispensable to the architect, surveyor, and indeed should be understood by every person in any way connected with building, or designing. Nor should the following be neglected, viz. scenography, which shews us how to direct the visual rays to every point, or part, of a pic and stereography, which enables us to represent solids on a plane, from geometrical projection ; whence their se veral dimensions, viz. length, breadth,
and thickness, may all be represented, and be correctly understood at sight, We suppose our readers to have some knowledge of geometry before they com mence upon this, or any other of the ab stract sciences which are founded there on. Should such, however, not be the ease, we beg leave to refer them to that head, where they will find sufficient in struction to enable them to prosecute their inquiries on the subject now before us.
An original object, is that which be comes the subject of the picture, and which is the parent of the design. Any plane figure may become an object, as may any of its parts, as a broken pillar, the ruins of a house, the stump or the branch of a tree; but we generally speak of objects as relating to entire figures re presented as solids, or to as much rural or other scenery as may be embraced un der an angle of 60 degrees formed by two lines meeting at the eye. This will explain why we are enabled to represent so great a number of distant objects, while the front, or fore-ground, will con tain, comparatively, but a very few : it being obvious, that as the lines forming the angle become more distant, the more may be included between them.
Original planes, or lines, are the sur faces of the objects to be drawn ; or they are any lines of those surfaces ; or it means the surfaces on which these ob jects stand.
Perspective plane is the picture itself, which is supposed to be a transparent plane, through which we view the ob jects represented thereon.
Vanishing planes are those points which are marked upon the picture, by supposing lines to be drawn from the spectators's eye parallel to any original lines, and produced until they touch the picture.
Ground plane is the surface of the earth, or plane of the horizon, on which the picture is supposed to stand.
The ground line is that formed by the intersection of the picture in the ground plane.
The horizontal line is the vanishing point of the horizontal plane, and is pro duced in the same manner as any other vanishing line, viz. by passing a plane through the eye parallel to the horizontal plane.