SENSE OF SIGHT. 



563 



lens and vitreous, so that when the rays reach the retina they are 

 brought to a focus. This is shown in Fig. 339, where the arrow 

 XY is projected upon the retina, forming the inverted image YX. 



The angle xnY is the visual or optical angle, also called the 

 angle of vision, and determines the size of the image upon the retina. 



If an object subtends an angle less than 50 seconds, it cannot 

 be seen, because the size of the image upon the retina would be 

 less than 3.65 p. ; the distance between the centers of two adjacent 

 cones being about 4 /*, each cone having a diameter of about 3 fjt. 



Retinal Images are Inverted. By reference to Fig. 339 it 

 will be seen that the retinal image is inverted ; the question nat- 

 urally arises, therefore, Why are not the objects which form these 

 images seen in an inverted position ? The answer to this question 

 is that objects are "seen" by the brain, and not by the retina; 

 that certain impressions are produced by the light upon the retina ; 

 and that these impressions are transmitted to the brain by the 

 optic nerve, and are there interpreted in the form of what is 



M 



FIG. 340. Diagram illustrating the projection of a shadow on the retina. 



called " sight." The brain has by long experience come to asso- 

 ciate the top of an object with the image which the top of the 

 object produces on the retina, so that although the upper end 

 of an object as the point of an arrow, for instance makes its 

 image below, and the lower end or, in the supposed instance, the 

 head of the arrow forms its image above, the brain sees the 

 arrow with its point up and the head down. Light which reaches 

 the retina is always referred by the brain to an object situated on 

 the opposite side. Thus light which reaches the right side of the 

 retina comes from the left, and that which reaches its left side 

 comes from the right. 



It is a curious fact that when an upright object makes an 

 upright image on the retina, the brain inverts the object, so that 

 it is seen in an inverted position. This is illustrated in Fig. 340, 

 for which and for the recital of the fact we are indebted to the 

 American Text-book of Physiology. If a card with a small pin- 

 hole in it is placed in front of a source of light and about three or 



