128 Prof. Emerson on the Perception of Relief. 



left eyes respectively ; the larger this angle, the more relief is 

 apparent, provided the eyes can unite the dissimilar images ; but 

 when by reason of distance this angle becomes nothing practi- 

 cally, and the rays are parallel as they enter the eyes, relief 

 vanishes. 



The perception of the perspective depends upon very different 

 conditions, such as the direction of the lines that compose a view, 

 the light and shade, the apparent size, the tint, &c. 



When we consider the matter, it is not surprising that these 

 two modes of perception should often be confounded. True 

 relief diminishes so gradually, and melts so gently away, leav- 

 ing perspective entirely master of the field, that the essential 

 difference between them is likely to be lost sight of. That this 

 is the case may be shown by the following examples : — 



It requires a series of very careful experiments to determine 

 how far, under ordinary conditions, we can perceive relief. Ex- 

 periments of my own lead me to believe that the distance is 

 under three hundred yards. The only reason a good painting, 

 whose foreground is represented as it appears at the distance of 

 two or three hundred yards, is not a complete illusion when seen 

 UDder favourable conditions, is that we can change our point of 

 view ; and motion to one side or the other will impart the idea 

 of relief in nature, but as there is no relief, properly so called, 

 in a painting, as soon as we shift the point of view, we detect 

 this, and the illusion is at an end. Hence paintings ought to be 

 observed by one eye, and from one point of view, to obtain the 

 maximum 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 

 photographed 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 

 cannot distinguish readily between stereoscopic and pseudoscopic 

 effect : they are also constantly imposed upon by views which 

 have 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 

 failed to get the verdict that they exhibited stereoscopic effect ; 

 which was impossible, of course. Not only are ordinary obser- 

 vers thus mistaken, but they constantly manifest an opposite 

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

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

 Leipsig, 1860, Dove's illustration of this point is republished in 



