PERSPECTIVE is the foundation of all the polite or liberal arts that have their basis in drawing; though colouring, taken abstractedly, does not come within its rules, yet the painter, as well as the sculptor and architect, cannot but de rive essential advantages from a know ledge of perspective ; it is indeed diffi cult to conceive how a person, who has not either been instructed in, or been gifted by nature with some idea of the effects produced by locality and distance, can form any thing like a correct opinion of the merits of those imitations of na ture which come under the heads of por trait, landscape, figure, or architectural drawing.
Perspective is, in brief, the art of re presenting, upon a plane surface, the ap pearance of objects, however diversified, similar to that they assume upon a glass plane, interposed between them and the eye at a given distance. The represen tation of a solid object on a plane surface can spew the original in no other point of view but that from which it is at the time beheld by the draughtsman; the least change in any of the parts requires a change in the whole ; unless in fancy drawings, where a facsimile is not re quired. Nor can any deviation from the several lines, which will be hereafter ex plained, and on which the truth and cor rectness of representation depend, be al lowed, without changing the bearings, di rections, and tendency of all the perspec tive lines, which constitute the basis of that faithful and converging series which unite all the component parts in the most pleasing and harmonious concinnity.
By perspective we are taught to deli neate objects on a plane, upon geometri cal principles, and in exact ratio with their several magnitudes, governed by their distance. But it is not in the power of art to represent any single figure, (exact as it appears in nature), on a plane, except it be a circle ; and then the point of sight, or direct position of the eye, must be perfectly centrical. The reasons for this are obvious ; every ob ject which recedes from the eye, (such as a row of houses in an oblique direc tion), inevitably requires that its more remote parts should be represented as being of less magnitude than those more in front, that is, nearer to the spectator. Now, although it is considered an axiom in perspective that all objects standing parallel to the base line, or bottom of the picture, should be represented as pre. serving in every instance the real propor tions of the scale from which their parts were taken ; yet when we analyze the object, according to the various angles those several parts make with the eye, we shall find that even such full pointing figures require their more remote parts to be reduced in proportion as they be come more distant from the centre, or point of sight. But it will be obvious, that where the object is very remote, there must be the less necessity for such scrupulous attention; therefore, when we draw an extensive mansion, full fronting, at a great distance, we describe all the horizontal lines in the building by hori zontal lines in the drawing • so long as they are comprehended within an angle of 60 degrees ; which is the natural range of sight, and beyond which no pic ture should ever extend ; when beyond that angle, we cannot take the whole pic ture at one view ; but must treat it as a panorama, and view the several parts ab stractedly. When a building is so near
as to occasion turning our heads round for the purpose of seeing its several parts, they have the same effect, and compel us to have recourse to various vanishing points, in which we seek the termination of those lines that converge, and in fact divide the building, though full fronted and uniform, into several parts; each of which seems to assume a distinct character, and to demand sepa rate consideration This will be more fully understood when we treat of the general rules which govern perspective. The reader must recollect, that, as it would be impossible to represent more than one view of the object, in one plane, or picture, so there can be but one point of sight ; that is, but one par ticular spot, where the eye of the spec tator is supposed to be fixed ; from which, as from a very minute point, all the figures represented must appear as under one general system. The same attention must of course be paid to shadows ; for we cannot suppose the dark side of a house to result from any thing but the light being in such a quarter as does not allow it to strike on that side ; consequently we attribute the bright side of the same object to its being illu minated by the rays which act peremp torily upon it. Speaking of common ef fects, we consider the light to be solita ry ; such as the Sun, or the Moon, or one candle, &c. ; hence we perceive both the necessity, and the reason, for ex hibiting all objects as bright, which are within the range of, or shew themselves openly to, the light, and all parts to which its rays cannot reach direct, as being in the shade and more or less dark accord ing as they may be more retired and con fined. When two lights are found in the same picture, such as two candles on a table, there will be to every object tin der their mutual influence a half shade, and a whole shade ; the former called the penumbra, shewing thaf extent which re sults from one light being obscured, or cut off; and the latter, or the umbra, showing those parts which are not acted upon by either of the lights. This will be obvious to any person who may place two candles behind him, as he sits with his back to a table ; they being about two feet asunder. He will then see, on the wall, the influence of each candle ; and his shadow will increase with the remote ness of the plane, or wall, on which it is represented.