I2 8 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



ray line to its proper place. Light from b passes through 

 the nodal point n and strikes a rod or cone in the upper 

 part of the retina, b', and is referred back along the ray- 

 line to b. Similarly light from e passes a little downward 

 through n and impresses the lower part of the retina, and 

 is referred back to its proper place below. Thus the 

 three stars will be seen in proper relative positions. Thus 

 the lower part of the retina corresponds to the upper 

 part of the field of view, and the upper part of the retina 

 to the lower part of the field; the right side of the retina 

 to the left side of the field, and the left side of the retina 

 to the right side of the field ; and in each case with the 

 utmost exactness, point for point. Every rod and cone, 

 as it were, knows its own point in space, and refers its 

 impressions there. 



Comparison with Other Senses. — Now, this is no 

 unique law peculiar to sight alone, but a general law of 

 sense-reference, though refined to the last degree in this 

 sense. In the case of any impression on a sensitive sur- 

 face the cause is referred in a general way along the line 

 of impression. Suppose we are captive, bound and blind- 

 folded, and surrounded on all sides by Apache Indians 

 and a target for their arrows, could we not tell roughly 

 the direction of each shooting Apache by the direction 

 of the punch of his arrow? Now, every radiant is shoot- 

 ing rays into the eye. Is it not natural that every im- 

 pinging ray should be referred back along the line of 

 its flight to the point whence it came ? Now, the retina 

 is specially and wonderfully organized to do this with 

 mathematical exactness. 



These two laws, the law of spatial reference and the 

 law of direction, are fundamental. The one explains 

 why impressions made in ourselves are referred outward 

 into space. The retina, or the brain through the retina, 

 creates visible space. The other gives the direction of 



