Emerson on the perception of Relief . 315 



L effect. Hence, also, stereographs of scenes which lie 

 at a distance of over three hundred yards from the observer will 

 give no stereoscopic effect, will not give the impression we are 

 able to get with our eyes, assisted by our capacity to move from 

 one point of view to another; they ought, therefore, to be pho- 

 tographed from stations more or less distant from each other, but 

 always exceeding considerably the distance between the eyes. 



Persons not accustomed to experimenting with the stereoscope 

 eaiiiiot distinguish readily between stereoscopic and pseudoscopic 

 •-ilbct ; they are also constantly imposed upon by views which 

 liave no stereoscopic effect whatever ; I have repeatedly mounted 

 two identical or right-eye views of the same scene, side by side, 

 as though they were right and left eye views and have never 

 &iled to get the verdict that thev exhibited stereoscopic effect; 

 which was impossible, of course". Not only are ordmary ob- 

 servers thus mistaken, but they constantly manifest an opposite 

 peculiarity, being unable to see the greatest relief when it 13 

 exhibited in an unusual manner. In Das Stereoscop, C. G. Kuete, 

 Leipsig, I860, Dove's illustration of this point is republished m 

 such a way as to destroy the object in view, showing that his 

 commentator had not a fine perception of relief. 



A remarkable instance of the uncertainty attending the per- 

 ception or non-perception of stereoscopic relief, even m cases 

 where we might suppose there could be no want of knowledge 

 IS shown by the controversy now going "" - t^— "^ ^^^^r h^ 

 Chimenti pictures. Sir David ^ ' "~ 



drawings produced about 

 this opir' 



ndorsed 



pictures a specimen of real stereoscopic 

 the middle of the 17th century ; and _ , , ■ 1 -, 

 b Prof. Tait, Prof. M'Donald and others m decided 

 I have made a careful examination of the photographs of these 

 pictures, and the truth is that the trifling stereoscopic and pseu- 

 <ioscophic qualities about them are evidently accidental T.o 

 prove this let any one execute a pen-and-ink sketch; and then 

 let him make as perfect a copy of it as he can without careful 

 cieasurements; now place these two drawings in the stereoscope 

 aid you get the same kind of effect seen in the Chimenti draw- 

 ings, and for the same reason; the drawings will vary more or 

 less from each other; all that is necessary then to impose iipon 

 ordinary eyes, is to find out which way the sum ol tfie^ variations 

 preponderates; mount the drawings accordingly, and, mirabile 

 •iictu! you have produced a stereoscopic picture ^(the pseuclo- 

 ^opic portion being overlooked) drawn by haiid; you have 

 ^one that very thing that Sir David Brewster has repeatedly 

 declared was quite biyond human skill! If Prof. Wheatstone 

 gets no heavief blow than this, his fame as a discoverer is secure. 

 , As a further confirmation of our views, we may point to the 

 fact that but few persons can properly locate the optical position 



