304 RECEXT PROGEESS OP THE THEORY OF VISION. 



in the diagram which correspond, and make them overlap 

 by a volmitary movement of the eyes. But as long as I 

 have not made out what kind of form the drawings are in- 

 tended to represent, I find that ray eyes begin to diverge 

 again, and the two points no longer coincide. Then I try 

 to follow the different lines of the figure, and suddenly I 

 see what the form represented is. From that moment my 

 two eyes pass over the outlines of the apparently solid 

 body with the utmost ease, and without ever separating. 

 As soon as we have gained a correct notion of the shape 

 of an object, we have the rule for the movements of the 

 eyes which are necessary for seeing it. In carrying out 

 these movements, and thus receiving the visual impres- 

 sions we expect, we retranslate the notion we have formed 

 into reality, and by finding this retranslation agrees with 

 the original, we become convinced of the accuracy of our 

 conception. 



This last point is, I believe, of great importance. 

 The meaning we assign to our sensations depends upon 

 experiment, and not upon mere observation of what takes 

 place around us. We learn by experiment that the cor- 

 respondence between two processes takes place at any 

 moment that we choose, and under conditions which we 

 can alter as we choose. Mere observation would not give 

 us the same certainty, even though often repeated under 

 different conditions. For we should thus only learn that 

 the processes in question appear together frequently (or 

 even always, as far as our experience goes) ; but mere 

 observation would not teach us that they appear together 

 at any moment we select. 



Even in considering examples of scientific observation, 

 methodically carried out, as in astronomy, meteorology, 

 or geology, we never feel fully convinced of the causes of 

 the phenomena observed until we can demonstrate the 

 working of these same forces by actual experiment in 



