VII 



KETINAL EXCITATION 



337 



the visual angle. The perception of objects of which the images 

 fall on the fovea is called direct vision, in contrast to indirect vision 

 from the peripheral parts of the retina. 



Indirect vision is less distinct, and we are unable by it to 

 recognise the outlines, forms, and minute details of objects, but it 

 enables us to distinguish changes and movements within the field 

 of vision, and guides and orientates us, so that we can avoid obstacles 



FIG. 162. Forster's perimeter. 



in walking (S. Exner). It becomes increasingly less acute and 

 more blurred in proportion as the images fall on the retina farther 

 from the fovea and nearer the periphery. 



The visual field does not coincide with the whole area of the 

 retinal surface. Its extension varies in different individuals, in 

 different meridians, and even to a slight extent in the two eyes, 

 under both normal and abnormal conditions. The field for white 

 is most extensive, and next in order come those for blue, red, and 

 green. 



The perimeter is used to determine the range of indirect vision, 

 VOL. iv z 



