392 AIM AND PROGRESS OF PHYSICAL SCIENCE. 



organisation by which external objects discourse to us 

 a language, however, like our mother tongue, that we 

 can only learn by practice and experience. 



Moreover, what has been said holds good not only for 

 the qualitative differences of sensations, but also, in any 

 case, for the greatest and most important part, if not the 

 whole, of our various perceptions of extension in space. 

 In their bearings on this question the new doctrine of 

 binocular vision and the invention of the stereoscope 

 have been of importance. All that the sensation of the 

 two eyes could convey to us directly, and without 

 psychical aid was, at the most, two somewhat different 

 flat pictures of two dimensions as they lay on the two 

 retinae ; instead of this we perceive a representation 

 with three dimensions of the things around us. We 

 are sensible as well of the distance of objects not 

 too far removed from us as of their perspective juxta- 

 position^ and compare the actual magnitude of two 

 objects of apparently unequal size at different distances 

 from us with greater certainty than the apparent equal 

 magnitudes of a finger, say, and the moon. 



One explanation only of our perception of extension 

 in space, which stands the test of each separate fact, can 

 in my judgment be brought forward by our assuming 

 with Lotze that to the sensations of nerve- fibres, dif- 

 ferently situated in space, certain differences, local signs, 

 attach themselves, the significations of which, as regards 

 space, we have to learn. That a knowledge of their 

 signification may be attained by these hypotheses, and 

 \\ith the help of the movements of our body, and that 

 we can at the same time learn which are the right move- 

 ments to bring about a desired result, and become 

 conscious of having arrived at it, has in many ways been 

 established. 



That experience exercised an enormous influence over 



