When the head and eyes are maintained in a fixed position, and one eye is closed, the area seen by that eye constitutes the visual field for that eye. Now if the other eye is also opened the area seen is more or less the same as was seen with one eye. In other words the visual fields of the two eyes overlap to a very great extent. On either side, however, there is a small area seen only by the eye of that side. Although the two eyes view the same area, the relative position of objects within the area appears somewhat dissimilar to the two eyes as they view the object from slightly different angles. The difference though slight, is of considerable importance as it forms the basis for the perception of depth (stereoscopic vision).
For convenience of description, the visual field is divided into right and left halves. It may also be divided into upper and lower halves so that the visual field can be said to consist of four quadrants (Fig. 18.3). In a similar manner each retina can also be divided into quadrants. Images of objects in
the field of vision are formed on the retina by the lens of the eyeball. As with any convex lens the image is inverted. If an object is placed in the right half of the field of vision its image is formed on the left half of the retina and vice versa. Unfortunately, instead of using the words right and left, the two halves of the retina are usually referred to as nasal (= medial) and temporal (= lateral) halves. This introduces a complication as the left half of the left eye is the temporal half, while in the case of the right eye it is the nasal half. Thus the image of an object placed in the right half of the field of vision falls on the temporal half of the left retina, and on the nasal half of the right retina.