690 



SPECIAL SENSES 



and the apparent increase in the size of the object probably is due to 

 the fact that its distance from the eye is many times less than the 

 distance at which distinct vision is possible under ordinary conditions. 

 It is well known that myopic persons, by being able to bring the eye 

 nearer to objects than is possible in ordinary vision, can see minute 

 details with peculiar distinctness. 



Erect Impressions produced by Images inverted tipon the Retina. 

 The images that make visual impressions are necessarily inverted on the 

 retina ; but the cerebral visual centre takes no cognizance of this, and 



objects are seen in their 

 actual position. It seems 

 almost absurd to enter 

 into a serious discussion 

 of this fact. Quoting 

 the words of von Helm- 

 holtz, "our natural con- 

 sciousness is completely 

 ignorant even of the ex- 

 istence of the retina and 

 of the formation of im- 

 ages: how should it know 

 anything of the posi- 

 tion of images formed 

 upon it ? " 



Field of Indirect Vis- 

 ion. If the eye is kept 

 fixed on a certain point, 

 and an object is moved 

 from this point as a 

 centre in lines radiating 

 in different directions 

 until it passes from the field of view, the limits of indirect vision are 

 indicated. Eight or ten such points of limit, connected by a curved line, 

 give a map of the visual field. This may be done roughly on a flat 

 surface, such as a blackboard, placed at a distance of twelve to eighteen 

 inches (3 to 4.5 centimeters) from the eye, or a chart may be made with 

 an instrument called the perimeter, by which the field is marked on the 

 inner surface of a hemisphere. The field of vision thus delineated is 

 an irregular oval, extending from the fixed point farther to the temporal 

 side than to either the nasal side or above and below. The extent from 

 the fixed point is about 90 on the temporal side, and about 70 to the 

 nasal side and above and below. The field for white light is larger than 



Fig. 177. Field of vision of the right eye, as projected by the 

 patient on the inner surface of a hemisphere, the pole of -which 

 forms the object of regard semidiagrammatic (Landolt). 



T, temporal side ; N, nasal side ; W, boundary for white ; B, 

 boundary for blue ; /?, boundary for red ; G, boundary for green. 



