FORMATION OF THE IMAGE UPON THE RETINA 851 



This image, therefore, would scarcely equal the diameters of two red 

 corpuscles and would cover about four cones of the fovea centralis. 

 This same object placed at a distance of one mile (5280 feet), would 

 give an image measuring 0.341 mm. in height, which corresponds to 

 about the diameter of the fovea centralis. 



The Visual Field. Perimetry. — If our attention is called to an 

 object, our eyes are always turned in such a way that its central area 

 is brought to a precise focal point in the foveae centrales. This act 

 constitutes direct vision. At this very time all other objects in space 

 fall upon the outlying districts of the retinae and are therefore seen by 

 indirect vision. Direct vision, therefore, is effected through the visual 

 axis, connecting the object with the fovea, and indirect vision through 

 secondary axes which fall upon the more peripheral zones of the retina. 

 Both eyes together cover a certain extent of the external world which 

 is known as the visual field, but this entire field is really made up of two, 

 a right and a left, the nasal spheres of which overlap. From what 

 has been said above regarding the manner of refraction in our eye, it 

 must be evident that the retinal image is inverted and that objects 

 situated in the upper extent of the visual" field, are centered upon the 

 lower half of the retina, and vice versa. The same is true of objects 

 situated respectively in the right and left halves of the visual fields, 

 because they fall upon the opposite side of the retina. 



The. configuration of the entire visual field, as well as of that of 

 each eye, depends chiefly upon the anatomical characteristics of the 

 margins of the orbital cavity. Centrally, each field is restricted by the 

 bridge of the nose, above by the orbital arch, and below by the cheeks. 

 Consequently, each field really presents an irregular oval outline, in- 

 stead of a circular one which it would possess if the eye were protruded 

 beyond these restricting boundaries. Its limits may be ascertained by 

 steadily gazing with one eye upon a mark upon a large cardboard, 

 placed at a distance of about 25 cm. vertically in front of the cornea. 

 The visual axis of this eye should strike the cardboard exactly at right 

 angles. A small object is then moved from without along the vertical, 

 horizontal and oblique meridians as charted upon the cardboard. A 

 mark is made each time when the observed person obtains a clear 

 impression of this object. If these outlying points are then joined 

 with one another, we obtain the boundaries of the visual field of this 

 particular eye at the distance of 25 cm. 



An instrument commonly made use of for mapping out the visual field is 

 the perimeter. The one devised by Aubert and Forster^ (Fig. 452), consists of a 

 hemispherical band of metal fastened to a stand and movable so as to cover the 

 different meridians of the eye. In front of this arc is placed a support for the chin 

 of the observed person, his eye being adjusted in such a way that he is able to gaze 

 horizontally at a white object fastened to the center of this circle (Fig. 452). 

 A small white disc is then moved slowly from without along this arc until it be- 

 comes clearly visible. The arc is graduated, allowing the moment of the appear- 

 ance of this object to be charted (Fig. 453). This procedure is repeated along the 



1 Archiv fiir Ophthalmologic, iii, 1857. 



