PER

282

PER

must be in the picture a gradual rising of the ground-linetowards a point horizontally opposite the place of the aper-ture :—Now the whole art of perspective consists in observ-ing rules which teach us to discover the diminutions of allobjects seen obliquely, like the gable-end of the house. Torender this experiment, and the inferences drawn from it,perfectly clear, it ought to be tried and fully considered. Itwill then speak to the eye, and the object to be obtained by! perspective can scarcely be misunderstood, whereas theimpression of mere words is speedily effaced.

To prevent any incorrect inference, we shall, however,refer to fig. 1, where, let c D represent the window-shutterof the darkened chamber, and g the aperture in it, a b anexternal object, and e 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 form an image from| any other; and that if the direction of the rays could beI ascertained as much before the shutter, as they are here! behind it, an image of the original object would be obtainedof the same size as that upon the screen, and in its erectposition, because the rays have not crossed. Accordinglyin the practice of perspective, the rays of light from an objectare always supposed to be intercepted as they converge tothe eye at some point, as at h, between the original objectand the eye. In the experiment, therefore, the aperture inthe window-shutter must be considered as representing thepupil of the eye, the darkened chamber of the eye, and thescreen the retina, or as a means of rendering visiblethe pictures which the eye receives of visible objects. Weneed not observe, that a larger aperture, with a convex glassj set in it, would, in fact, form a camera obscura, and a very| j distinct image would be painted on the screen, at the focus[ of the glass, but the experiment would then be less simple,

; and 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 pointof view, let A b c d, fig. 2, represent a house, seen by the eyeat N. The eye n, is supposed to be opposite the corner q ofI the house ; its distance from which is equal to N q, and itsj height five feet from the ground. The situation of the eyej corresponds to that of the hole in the window-shutter of the! former 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 partsrespectively, and thereby render the house visible. The eye,it must be understood, is fixed upon the point q, directlybefore it, and in order that no sensible deviation may bepossible, we may suppose it to be looking through a verysmall aperture in a piece of thin brass, g. If now a trans-parent plane, for example a pane of glass, k l, be interposedbetween the house and the eye, at a short distance from the eye,the whole of the house will be seen through the transparentplane, although the latter is, comparatively with the house,of very small dimensions, because the rays, in proceeding totheir point of convergence at the eye, have approached eachI other in a proportion inversely as the distance ; that is, athalf the distance from the object, they only extend over halfthe space contained between the points of emission ; at one-fourth of the distance from the eye, they only take up one-fourth of the space; and the same proportion for other dis-tances. Suppose the pane of glass to be within arm’s reachof the eye at n, and that it is coated with gum-water orisinglass, so as to receive the marks of a pencil, withouthaving its transparency destroyed j trace the outlines of the

the

house upon the glass, by observing and following exacdirection in which they are seen through the small opin the piece of brass. When this is done, it will f\ e .that the real or measured extents, forming the <11 3Cte ntsexternal surfaces of the house, are represented by ® ^modified by the distance and obliquity of these sur 0 fthe eye ; in short, as shown in the figure, a representthe house in true perspective will be obtained, in ,“y ofsituation of the eye. To young persons, the dim c iunderstanding an explanation of this kind is occasiotheir indistinct perception of the relation between jj n e3and lines from a real object, and the projection ol tl10 c oD'upon a flat surface, as a sheet of paper. It app ea ^ gfusing to them to say that the eye is opposite to the c< ^j jl j e gsof the house, and yet to represent it at n on one side.this difiiculty be overcome, and the mind can form a -wouldimage of the direction which the line shown on P a P®have if drawn from a real object, perspective ' ® ofwill be contemplated with pain, and the rememm* e 9them will soon be effaced. We shall therefore P‘ Plittle experiment, which we recommend to be tried Jwho feel the difficulty alluded to. ^ an d t0

Let a small model of a house be made of ,Te 0 situ<’ievery corner of it which can be seen in any one ^oilu 1affix a thread of silk, two or three times as long aS * ‘ +lie pi}' 3

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t)»e

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of the house is high. These threads will representof light proceeding from the corners A B C f 1house in Figure 2. Let the threads be drawn throUo^^,in a piece of thin brass, just large enough to ^il/rVl'be moved freely. The hole in the piece of brass w ^ eJ a i'sent the eye. Let small weights be attached to the ® ^ jiol®ties of the threads which have been passed throug fortu 9in the brass ; the threads being thus stretched, ' V1 g ivh 1right line from the house to the brass, and the app a ‘ -yyiiil®be ready for elucidating the nature of perspective-^.^ othe model of the house remains stationary, let the P oD]e tiiU e ®the brass be varied, sometimes placing it highef’ . g ick 9 ’lower, at different distances, and towards j tfr - tua tio n , jand let the angles formed by the threads in each sl e p’belu aattentively considered, by the observer placing *‘' inS e a s if 'the brass, and supposing himself to regurd the h 0 f t‘saw it through the hole. Let him after each ,em j lig' 1 !

ah

oiu

brass, suppose that the threads representingwithout altering their direction, were to pass t f 1 ^° Uc j[ )ra 5S fof paper, interposed at any distance between tb® t o jtiie house, and he would find that by drawing g f tbthe points thus obtained, an outline represent® ^, 0 uld ^house would be produced, and this representatio jj U

in true perspective. For any one

situation

Xped°unonT “, atter t0 Pirate a piece of P«P'other sitnat- ' 6 tllreads without distorting thems- aIr T 8 ’ 3 g °° d idea of wbat the representation! iJtl/

given points r "n ?’ ° f tho P ers P eclive s P ace f

given points, would be obtained by measuring t he

-■—♦he brass - ;f . fl .jjl

1

aonearf, t0 ^ 3 tolera % correct idea of the F—j ^

ppearance of any object, or assemblage of object* ‘ t ;vs

- Tn pew tli«

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to

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between the threads at equal

be obtained by measuri o 9>at equal distances 110™^ .^, w,

the trial and proper consideration of this e *P®j je pei'SP 1

In P‘

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difficult to exhibit that appearance on paper,diagrams, lines must be drawn to represendirection of which in this experiment is indica . t jaand as the view of an object varies fromit is seen, the situation of the eye, both m he o pfii s Pmust be laid down upon paper, on which _p 0 s e q’b®drawing of an object is to be made, unless we P pl a ° e ' e yeat the object itself as through a transpare^ ^ ([isquestion then occurs, how shall the pos* 11