Prof. Emerson on the Perception of Relief. 129 



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 in cases 

 where we might suppose there could be no want of knowledge, 

 is shown by the controversy now going on in Europe over the 

 Chimenti pictures. Sir David Brewster thinks he has in these 

 pictures a specimen of real stereoscopic drawings produced about 

 the middle of the 17th century; and this opinion is endorsed 

 by Prof. Tait, Prof. Macdonald, and others in decided terms. 

 I have made a careful examination of the photographs of these 

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

 doscopic qualities about them are evidently accidental. To 

 prove this, let anyone execute a pen-and-ink sketch, and then 

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

 measurements : now place these two drawings in the stereoscope, 

 and 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 upon 

 ordinary eyes is to find out which way the sum of the variations 

 preponderates ; mount the drawings accordingly, and, mirabile 

 dictu ! you have produced a stereoscopic picture (the pseudo- 

 scopic portion being overlooked) drawn by hand; you have 

 done that very thing that Sir David Brewster has repeatedly 

 declared was quite beyond human skill ! If Prof. Wheatstone 

 gets no heavier 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 

 of reflexions from curved surfaces, and in particular the images 

 from concave surfaces. 



During the last year or two, large assemblages have been 

 drawn together in our principal cities to see with delight the 

 effects produced by what is called the Stereopticon, which is 

 merely another name for a magic lantern of good quality, with 

 one side of a glass stereograph for a slide. Nearly all in these 

 large assemblages have agreed in believing that they saw, what 

 they were told they saw, excellent stereoscopic effect in the single 

 picture which alone is exhibited. The truth is they made the 

 popular mistake ; they saw nothing but perspective. 



Stereoscopic effect on a large scale may be obtained by exhi- 

 biting the right and left pictures of a glass view side by side by 

 the magic lantern, and then uniting the magnified pictures by 

 means of prisms. This I have recently demonstrated by expe- 

 riment. The idea was also suggested some years ago by Dr 

 Wolcott Gibbs to Mr. Pike of New York, but not put to the 

 test. 



Phil. Mag. S. 4. Vol. 25. No. 166. Feb. 1863. K 



