FIRST PRINCIPLES 39 



flat picture with a certain incidence of light and shade ; but 

 the two pictures are not the same. The right eye sees more 

 of the right of the ball, the left more of the left. Each 

 picture is clear in outline, yet when the two pictures are 

 superposed it is only at the two poles of the ball that the out- 

 lines of shading coincide. Yet to those who have always 

 enjoyed the sense of sight the two eyes do not give a blurred 

 picture of a spherical object, even though it be illuminated 

 but for an instant by a flash of lightning, but one clear in 

 outline, and that not the picture of a flat disc but of a solid 

 sphere. It is not the eye, but the finger, that has taught us 

 that the ball is solid. We have learnt to associate the super- 

 position of two non-coinciding retinal images with the 

 extension in three dimensions of an object. And so well 

 has our Mind learnt this lesson that, instead of seeing a 

 blurred picture, we see a clear-cut presentation of a sphere. 

 Artists know that in painting a round ball they must progres- 

 sively increase the blurring of the lateral outline from the 

 poles to the equator ; but it is dangerous to go far in this 

 attempt to delude the eyes, since it can only produce the 

 right result when viewed at one particular distance from the 

 canvas ; and even at the right distance the two eyes soon 

 find out the fraud. The brain, paying attention in rapid 

 alternation to the images transmitted through the right and 

 left eye respectively, discovers that they are both blurred, not 

 clear when viewed separately, and blurred when superposed. 

 A seascape painter is reported to have said that the compli- 

 ment to his artistic skill which he felt most keenly was paid 

 him by an uncultured country friend. Attracted to his 

 studio by a heavy thud upon the floor, he entered just in 

 time to see his friend's boots projecting through the canvas 

 of his last and most successful picture of a deep, clear, 

 sun-lit pool. So perfect an illusion had his art produced 

 that his friend had given way to a natural impulse and 

 "taken a header." It requires but little physiological 

 knowledge to enable one to draw the conclusion that the too 



