POPULAR SCIENTIFIC LECTURES. 



007 



pTicr, gives on the retina, which is its sensi- 

 tive plate, only perspective views of the ex- 

 tf rnal world ; these are stationary, like tbo 

 c:iu\ving on a picture, as long as the stand- 

 po : "t oi! tbe eye is iiot altered. And, in fact, 

 ill >.e restrict cmrselves in the first place to 

 tao 1'orui of tho object viewed, and disregard 

 f"r Li:* present any consideration of color, 

 I ;, a (Mrieot. perspective drawing we can pre- 

 t:ut t ..) tho o\c of an observer, who views it 

 fr>'-u a correctly chosen point of view, tho 

 frtia.- i'orms of the visual image as the inspec- 

 tion of tho objects themselves would present 

 to tii<- sf.iiio eye, when viewed from the corre- 

 Kpoii'ihu,' point of view.' 



lint iipurt from the fact that any movement 

 of (ho observer, whereby his eye changes its 

 position, will produce displacements of the 

 visual iantgo, different when he stands be- 

 fore objects from those when he stands be- 

 fore the image, I could speak of only one eye 

 for which equality of impression is to be es- 

 tablished. We however see the world with 

 /ic<> eyes, which occupy somewhat different 

 positions in space, and which therefore show 

 two different perspective views of objects 

 before us. This difference of the images of 

 the two eyes forms ono of the most impor- 

 tant means of estimating tho distance of ob- 

 jects from our eye, and of estimating depth, 

 and this is what is wanting to the painter, or 

 even turns against him ; since in binocular 

 vision the picture distinctly forces itself on 

 otir perception as a plane surface. 



You must all have observed the wonderful 

 vividness which the solid form of objects ac- 

 quires when good stereoscopic images nro 

 viewed in the stereoscope, a kind of vivid- 

 ness in which either of the pictures is waiit- 

 ng when viewed without the stereoscope. 

 Che illusion is most striking and instructive 

 dth figures in simple line ; models of crys- 

 ,als and the like, in which there is no other 

 element of illusion. The reason cf this de- 

 ception is, that looking with two eyes we view 

 the world simultaneous!}' from somewhat 

 different points of view, and thereby acquire 

 two different perspective images. With the 

 right eye we see somewhat more of the right 

 side of objects before us, and also somewhat 

 more of those behind it, than we do with 

 the left eye ; and conversely we see with the 

 left, more of the left side of an object, and 

 of the background behind its left edges, and 

 partially concealed by the edge. B\it a flat 

 picture shows to the right eye absolutely tho 

 pamo picture, and all objects represented 

 upon it, as to the left eye. If, then, we make 

 for each eye such a picture as that eye would 

 perceive if itself looked at the object, and if 

 both pictures are combined in the stereo- 

 scope, so that each eyo sees its corresponding 

 picture, then, as far as form is concerned, tho 

 same impression is produced in the two eyes 

 as the object itself produces. But if we look 

 at a drawing or a pictxire with both eyes, wo 

 just as easily recognize that it is a represen- 

 tation on a plane surface, which is different 

 from that which the actual object would 

 show simultaneously to both ejes. Hence is 



(Tr.e the well-known increase in the vividness 

 of a picture if it is looked at with only ono 

 eye, and while quito stationary, through jv 

 dark tube ; we thus exclude any comparison 

 of its distance with that of adjacent objects 

 in the room. For it must be observed th?.t as 

 we use different pictures seen with the <tvro 

 eyes for the perception of depth, in liko 

 manner as the body moves from one place to 

 another, the pictures seen by the same eyo 

 serve for the same purpose. In moving, 

 whether on foot or riding, the nearer objects 

 are apparently displaced in comparison witli 

 the more distant ones ; the former appear to 

 recede, the latter appear to move with ns. 

 Hence arises a far stricter distinction between 

 what is near and what is distant, than seeing 

 with ono eye from one and the same, spot 

 would ever afford us. If we move toward 

 the picture, the sensuous impression that it 

 is a flat picture hanging against the v/all 

 forces itself more strongly upon us than if wo 

 look at it while wo are stationary. Compared 

 with a large picture at a greater distance, all 

 those elements which depend on binocular 

 vision and on the movement of the body aro 

 less operative, because in very distant objects 

 the differences between the images of the 

 two eyes, or between the aspect from adja- 

 cent points of view, seem less. Hence largo 

 pictures furnish a less distorted aspect of 

 their object than small ones, while tho im- 

 pression on a stationary eye, of a small pic- 

 ture close at hand, might be just the samo as 

 that of a lar^e, distant one. In a painting 

 close at hand the fact that it is a ilat picture 

 continually forces itself more powerfully and 

 more distinctly on our perception. 



The fact that perspective drawings which 

 are taken from too near a point of view may 

 easily produce a distorted impression, is, I 

 think, connected vilh this. For here tho 

 want of the second representation for the 

 other eye, which would be very different, is 

 too marked On tho other hand, what aro 

 called geometrical projections, that is, per- 

 spective drawings which represent a view 

 taken from an infinite distance, give in many 

 cases a particularly favorable view of the ob- 

 ject, although they correspond to ft point of 

 sight which does not in reality occur. Hero 

 the pictures of both eyes for such an object, 

 are the same. 



You will notice that in these respects thero 

 is a primary incongruity, and one which can- 

 not bo got over, beween the aspect of a picture 

 and the aspect of reality. This incongruity 

 may be lessened, but never entirely over- 

 come. Owing to the imperfec^ action of bi- 

 nocular vision, the most important natural 

 means is lost of enabling the observer to es- 

 timate the depth of objects represented hi 

 the picture. The painter possesses a series of 

 subordinate means, partly of limited ap- 

 plicability, and partly of slight effect, of ex- 

 pressing various distances by depth. It is 

 not unimportant to become acquainted with 

 these elements, as arising out of theoretical 

 considerations ; for in tho practice of the art 

 of painting tiny Lave manifestly exercised 



