Let a small model of a house be made of wood, and to every corner of it which can be seen in any one situation, affix a thread of silk, two or three times as long as the model of the house, is high. These threads will represent. the rays of light proceeding from the corners x n C k 1 at of the house in Figure..2. Let the threads lie drawn through a hole in it piece of thin brass, just large enough to admit them to lie moved freely. The hole in the piece of brass %Yin repre sent the eve. Let sinall weights be attached to the extremi ties of the threads which have been passed th•outdi the hole in the brass; the threads being thus stretched, will form a right line from the house to the brass, and the apparatus will be ready for elucidating the nature of perspective. \\idle the model of the house remains stationary. let the position or the brass be varied, sometimes placing it lihdier, sometimes lower, at different distances, and towards ehalirent and let the angles roomed by the threads in each situation be considered, by the observer placing himself behind the brass, and supposing himself to regard the house as if he saw it through the hole. Let him after each remove or the brass, suppose that the threads representing the rays of light, without altering their direction, were to is through a sheet of paper, interposed at :my distance between the brass and the house, and lie would find that by drawing lines to join the points thus obtained, an outline representation of the house would be produced, and this representation would be in tine perspective. For any one siltation. it would not be a troublesome matter to perforate a piece of paper. to lie slipped upon the threads without distorting them ; and Gir other situations. a good idea of what the representation would be, or, in ()Idler words, of the perspective space between given points, would he Obtained by measuring the openings between the threads at equal distances from the brass. After the trial and proper consideration of this experiment. it will he easy to rorm a tolerably correct idea of the perspective almearance of any objept, or assemblage of objects, and not difficult to exhibit that appearance on paper. In perspective diagrams, linos must be drawn to represent the rays, the direction of %illicit in this experiment is indicated loy threads, and as the view of an object varies froin the point in which it is seen, the situation) of the eye, both in height and distance, must be laid down upon paper, on which the perspective drawing or an object. is to be made, unless xi e propose to look at the object itself' as through a transparent plane. The question then occurs, how shall the position of the eye be designated on paper ? It can no way be represented so clearly as li• placing it on one side, as shown in the figure, or by placing it ‘ertiva ly beneath the object to be drawn, as presented in Figure 12.
By whatever means the representation t u, Figure 2. of all
object A It C 1), is Obtaillcd, if the outline be accurate, and viewed at a proper distance, it is pail it will make an out line of the same ferm in the eye as the object itself; and if the coiooriwg were equally perfect, the eye might mistake the flit. the original. But even when colours are not em ployed. correct dimensions give the whole a pleasing ap pearance, and constitute the first great requisite to every good picture.
llaving thus endeavoured to explain the nature of perspec tive, we may next. advert to the limits of vision. We may consider the eye, in w hatever direction e look, as situated in the centre of a sphere, which we, may suppose to be repre sented by the circle x F I, Figure 3. The henrisiihe•e z L F is behind the et e, and therefore obviously invisible; and it is also certain, that the eye, looking forward horizontaly to x, cannot take in at once the whole of the hemisphere E K F. So far from this, it cannot take in a larger angle than s x T, which is hemisphere, or equal to 90 degrees. And as the ray s which the eye takes hi, extend all round to an (vial distance from the central ray ii tt, it follows that the whole of the rays which enter the eye at once, will be in the form of a cone, of which the apex is at the eye ; and of such a cone of rays. s e T may be considered as the profile. It is however that to have an agreeable view of large ob jects, such as buildings, the angle of vision should not exceed 60 degrees. or one-third it' an hemisphere; in other words, that we catunit distinctly see the whole of an id jut, unless its distance from the eye he at least equal to its height ; and the appearance of a picture will be wore agreeable, if not made to comprehend above 45. or at must 50 degrees; in deed, for small objects, or such as do not exceed the length of a foot in ally of their dimensions, it is rot advisable to exceed an angle of 30 degrees. As a picture therefore should lever comprise more than the eye can easily take in at one view, a distance of 25 degrees on either side of the point of sight, may be considered a standard limit. Fifty degrees to the eve at B, are comprehended in the angle x a y; and we need scarcely observe, that the measure of all angle, is the space it takes up on the circumference of a circle, which has the point of the angle for its centre ; a circle being always supposed to contain 3i;0 degrees. Bence, if the lines 'limn ing the angle .r a y were extended, the angle vii t would still be only one of fifty degrees, because, whatever were the size of a circle drawn fawn the point a, through its two legs, if' that (ilex, were divided into 360 baits, the number of those parts enclosed by the angle, could not be more than fifty.
NVe snail now proceed to the definition of the terms used in treating of perspective, and then show the method of put ting into perspective, those forms which may be considered as the eleiBelits of all others,