THE MIND'S EYE. 305 



a table a figure (Fig. 4) made of light cardboard, fastened to blocks 

 of wood at the base so that the pieces would easily stand upright. 

 The middle piece, which is rectangular and high, was placed a little 

 in front of the rest of the figure. The students were asked to de- 



Fig. 7.— This is a highly enlarged reproduction taken from a half-tone process print of Lord 

 Kelvin. It appeared in the Photographic Times. 



scribe precisely what they saw, and with one exception they all de- 

 scribed, in different words, a semicircular piece of cardboard with 

 a rectangular piece in front of it. In reality there was no half-circle 

 of cardboard, but only parts of two quarter-circles. The students, 

 of course, were well aware that their physical eyes could not see what 

 was behind the middle cardboard, but they inferred that the two 

 side pieces were parts of one continuous semicircle. This they saw, 

 so far as they saw it at all, with their mind's eye. 



There is a further interesting class of illustrations in which a 

 single outward impression changes its character according as it is 

 viewed as representing one thing or another. In a general way we 

 see the same thing all the time, and the image on the retina does 

 not change. But as we shift the attention from one portion of the 

 view to another, or as we view it with a different mental conception 

 of what the figure represents, it assumes a different aspect, and to our 

 mental eye becomes quite a different thing. A slight but interesting 

 change takes place if we view Fig. 5 first with the conception that the 

 black is the pattern to be seen and the white the background, and 

 again try to see the white as the pattern against a black background. 

 I give a further illustration of such a change in Fig. 6. In our first 



