THE FUNCTION OF THE RETINA IN VISION. 853 



Faintly illuminated objects are not appreciated with the same degree of 

 accuracy by the fovea centralis as by the surrounding retina. 



If light be allowed to fall on the fovea centralis through a screen perforated 

 like a sieve, it appears as a continuous bright surface, if one point of light falls on 

 each cone. For this it is necessary that from 140 to 149 points of light fall on 

 o.oi sq. mm. of the fovea centralis. According to Salzer there are 138 cones 

 in this area. If the points of light in the screen are to be appreciated separately, 

 each illuminated cone must be surrounded by a circle of nonilluminated cones. 

 In this case 72 points of light must fall on o.oi sq. mm. of the fovea. 



To test the visual acuity in direct vision, two fine parallel lines drawn close 

 together, are gradually removed further from the eye, until they appear to fuse 

 almost into one. From the distance between the two lines, and the separation 

 of the drawing from the eye, the size of the retinal image is determined, and also 

 that of the corresponding visual angle, which is ordinarily between 60 and 90 

 seconds the lowest limit has been found to be between 50 and 27 seconds. 



Indirect vision occurs when the rays of light from an object fall upon 

 the peripheral portions of the retina. Indirect vision is much less sharp 

 than the direct, but the periphery of the retina has a well-developed 

 power of recognizing movements, changes or intermissions in visual 

 impressions. 



Perimetry. For the determination of indirect vision the perimeter of Aubert 

 and Forster is used. The eye is placed opposite a fixation -point, from which 

 extends a semicircle, so that the eye lies at its center. As the semicircle can be 

 revolved about the fixation-point, the surface of a hemisphere is formed by this 

 rotation, in the center of which the eye is placed. An object is now pushed outward 

 from the fixation-point along the semicircle toward the periphery of the visual 

 field, until it becomes indistinct and finally disappears. This test is made along 

 the various meridians by moving the arc into corresponding positions. The 

 further away from the fixation-point two closely placed points are carried, the 

 further they must be separated, to prevent their being fused. The power of 

 distinguishing colors diminishes more rapidly in the periphery than does that for 

 distinguishing differences in brightness. The decrease is, moreover, more marked in 

 the vertical meridian of the eye than in the horizontal, and decreases with the 

 distance from the fixation-point. Aubert and Forster discovered the remark- 

 able fact that in accommodation for a distant object the decrease of the differentiat- 

 ing power in the periphery occurs more rapidly than in accommodation for near 

 vision. The sensibility of the retina for colors and for brightness is greater at 

 points on the temporal side of the fovea than at equidistant points on the nasal 

 side. 



If the arc of the perimeter be divided into 90 degrees, commencing at the 

 fixation-point (Fig. 299) and proceeding to L and M, and if a series of concentric 

 circles be drawn about the fixation-point, a topographical chart of the visual power 

 can be mapped out for the normal and the diseased eye. Fig. 299 will serve as 

 an example. The thick lines refer to a diseased eye; the corresponding fine lines 

 to a normal one. The continuous line represents the limit for the perception of 

 white; the interrupted line, that for blue; the dotted and interrupted line that for 

 red (m is the blind spot, according to Hirschberg). The limits for the normal 

 eye are as follows: 



FOR WHITE. BLUE. RED. GREEN. 



Outward, 70-88 65 60 40 



Inward 50-60 60 50 40 



Upward 45-55 45 40 3 -35 



Downward, 65-70 60 50 35* 



The rods and cones alone possess the specific energy of being thrown 

 by the vibrations of the luminiferous ether into the activity that is 

 designated sight. Nevertheless, mechanical and electrical stimuli 

 applied to any portion of the course of the nervous apparatus can also 

 produce sensations of light. The mechanical stimulation is more 

 intense than that produced by light-rays, as is shown by the fact that, 



