252 RECENT PKOGRESS OF THE THEORY OF VISION. 



in place of the two ordinary photographs. They will then unite 

 into a single completely 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 copperplate 

 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 stereoscope, some lines will appear nearer and 

 some further off than the rest. This is the easiest way of de- 

 tecting spurious bank notes. A suspected one is put in a 

 stereoscope 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 experiment is also im- 

 portant 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 come to the question how is it possible for two 

 different flat perspective images upon the retina, each of them 

 representing only two dimensions, to combine so as to present a 

 solid image of three dimensions. 



We must first make sure that we are really able to distinguish 

 between the two flat images offered us by our eyes. If I hold 

 my finger up and look towards the opposite wall, it covers a 

 different part of the wall to each eye, as I mentioned above. 

 1 Accordingly I see the finger twice, in front of two different places 

 on the wall ; and if I see a single image of the wall, I must see 

 a double image of the finger. 



Now in ordinary vision we try to recognise the solid form 

 of surrounding objects, and either do not notice this double 

 image at all, or only when it is unusually striking. In order 

 to see it we must look at the field of vision in another way in 

 the way that an artist does who intends to draw it. He tries 

 to forget the actual shape, size, and distance of the objects that 

 he represents. One would think that this is the more simple 



