THE PERCEPTION" OF SIGHT. 287 



The accuracy of the stereoscope is no less wonderful. 

 Dove l has contrived an ingenious illustration of this. 

 Take two pieces of paper printed with the same type, or 

 from the same copper-plate, and hence exactly alike, and 

 put them in the stereoscope in place of the two ordinary 

 photographs. They will then unite into a single com- 

 pletely flat image, because, as we have seen above, the 

 two retinal images of a flat picture are identical. But 

 no human skill is able to copy the letters of one cop- 

 perplate on to another so perfectly that there shall not 

 be some difference between them. If, therefore, we print 

 off the same sentence from the original plate and a copy 

 of it, or the same letters with different specimens of the 

 same type, and put the two pieces of paper into the ste- 

 reoscope, some lines will appear nearer and some further 

 off than the rest. This is the easiest way of detecting 

 spurious bank notes. A suspected one is put in a stereo- 

 scope along with a genuine specimen of the same kind, 

 and it is then at once seen whether all the marks in the 

 combined image appear on the same plane. This ex- 

 periment is also important for the theory of vision, since 

 it teaches us in a most striking manner how vivid, sure, 

 and minute is our judgment as to depth derived from 

 the combination of the two retinal images. 



We now corne to the question how is it possible for 

 two different flat perspective images upon the retina, 

 each of them representing only two dimensions, to com- 

 bine so as to present a solid image of three dimen- 

 sions. 



1 Heinrich Wilhelm Dove, Professor in the University of Berlin, author 

 of Optische Studie.n (1859); also eminent for his researches in meteorology 

 and electricity. 



His paper, Anwendung des StereosJcops umfalschcs von cchtcm Papiergdd 

 tu unterscheiden, was published in 1859. TB. 



