266 RECENT PROGRESS OF THE THEORY OF VISION. 



case, I look out two points in the diagram which correspond, 

 and make them overlap by a voluntary movement of the eyes. 

 But as long as I have not made out what kind of form the drawings 

 are intended to represent, I find that my 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 

 impressions 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 con- 

 ception. 



This last point is, I believe, of great importance. The mean- 

 ing 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 correspondence between two pro- 

 cesses takes place at any moment that we choose, and under con- 

 ditions 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 the laboratory. So 

 long as science is not experimental it does not teach us the know- 

 ledge of any new force. 1 



1 An interesting paper, applying this view of the ' experimental ' character 



