In the following list will be found the names of some of the principal writers on perspective, with the dates of their perfilrinances, down to a comparatively modern period. Some of these authors have been already mentioned ; but in addition, several mathematicians of eminence have writtc;t on perspective; the latter have treated the art as the subject of pure geometry, as it really is. The performances of Dr. I;rook Ta? lor, Gravesande, Wolf, De la Caille, and Emerson, especially, are truly valuable, from the perspicuous simplicity and universality with which they have treated the subject.
ninst lie in the picture a gradual rising of the ground-line towards a point horiz.mtally opposite the place of the aper ture :—Now the Whole art of perspective consists in observ. Mg rules which teach us to discover the diminutions of all objects seen obliquely. like the gable-end of the house. To render this experiment. and the inferences drawn from it, perfectly clear, it ought to be tried and fully considered. It ‘N ill speak to the eve, and the object to be obtained by perspective can scarcely be misunderstood, whereas the impression Of mere wotd.s is speedily effaced.
To prevent any incorrect inference, we shall, however. refer to lig. 1, where, let c a represent the window.shutter of the darkened chamber. and q the apertinv in it, A B all eXtelaild Object, and F the screen which receives its image. It must be observed, that the darkened chamber is used only as a means of separating the rays which firm all linage from any other; and that if the direction or the rays conic] be aseertaine1 as much before the shutter. as they are here behind it, an image of the original object would be obtained or the same size as that upon the screen, and in its erect position, tweause the rays have not (Tossed. Acconiiincly in the practice or perspective, the rays of light flow nn oloject him' always supposed to be intercepted as they converge to the eye at some point, as at Ir, between the original object and the eye. In the experiment, theretiore, the aperture in I he \N liniSt lie considered as representing the pupil of the eye, the darkened chamber of the eye, and the screen the retina, or as a means of rendering visible th• pictures which the eye receives of visible ed,joetss \Vle need not observe. that a larger aperture, with a convex glass set in it, would. in l'act, form a eamera obscu•a, and a very distinct image would he painted on the screen, at the tiocus of the glass, but the experiment would then be less simple. anti the direction of the rays not so evident. Without a glass, the distinctness of the picture is sufficient to be agreeable, when the eye has been sometime in the chamber.
To consider the foundation of perspective in another point or \Jew. het A B C fig. '2. represent a house, seen by the eye at N. The ey e N, is supposed to be opposite the corner q of the house; ; its distance l'itom which is equal to x q, and its height five feet froth the ground. The situation of the eve
corresponds to that of the hole in the window-shutter of the tonne'. experiment, and the picture of the house formed in the eye itself corresponds to that which the screen received. In this situation, as in every other, straight lines drawn from every part of the house to the eye, represent the direction of the rays which form the images of those parts respectiu and thereby render the lionise visible. The eye, it must be understood. is fixed upon the point q, directly before it, and in order that no sensible deviation may lie possible, we may suppose it to be looking through :I very small aperture in a piece of thin brass, q. It' now a trans parent plane, for example a pane. of K L. he interposed the house and the eye, at a short distance the eye, the whole of the house will he seen through the transparent, plane, although the latter is, comparatively with the house. of very small dimensions, because the rays, in proceeding, to their point of convergence at the eye, have approached each other in It proportion inversely as the distance; that is. at hall the distance from the they only extend over hall' the space contained between the points of emission ; at mic ron. 1 li of the distance from the eye. they only take up one fourth of the space; and the sane proportion for other dis tan•es. :•;uppose the pane of glass to be within arm's reach of the eye at s, and that it is coated with gum-water or isinglass, so as to receive the marks of a pencil, without naving its transparency destroyed ; trace the outlines or the house upon the glass. by observing and fidlowing exactly the direction in which they are seen through the small aperture in the piece or brass. When this is done, it will be round that the real or measured extents, forming the different surfaces or the house. are represented by extents modified by the distance and obliquity of these surraces to the eye ; in short, as shown in the figure. a representation of the house in true perspective \rill be obtained, in the given situation of the eye. l'o oolong persons, the difficulty of understanding an explanation of this kind is (occasioned by their indistinct perception of the relation between the rays and lines rrom a real object, and the projeetio n of those lines upon a lint surface, as a sheet of paper. It appears con fusing to them to say that the eye is opposite to the corner q of the house, and yet to represent it at N on one side. Unless this difficulty be overcome, and the mind can firm a distinct image of the direction which the line shown on paper would have if drawn from a real object, perspective diagrams will be contemplated with pain, and the remembranee of them will soon be elliteed. We shall therefore propose a little experiment, which we recommend to be tried by those who feel the ollfficult v alluded to.