838 THE SENSE OF SIG^T 



be seen, therefore, that the visual appreciation of our external world 

 is accomplished by direct and indirect vision. The former leads to the 

 formation of an image in the fovea, and the latter to the formation of 

 an image upon the more outlying districts of the retina. 



This discussion clearly shows that the visual axis and the optical 

 axis of the eye are two distinct factors. The former is the imaginary 

 line along which the most distinct vision is obtained, because it con- 

 nects the object with the most sensitive area of the retina, the fovea 

 centralis of the yellow spot. The latter is the line of most perfect 

 refraction, because the different refractive media of the eye are centered 

 upon it. The most ideal system, of course, is the one in which the 

 refractive media are adjusted in such a way that their central rays are 

 brought to a precise focus upon the most sensitive region of the re- 

 ceptor. This is true of the photographic camera, but not of the 

 human eye, because the latter frequently shows a divergence of 

 its visual and optical axes of from 3.5 to 7 degrees. 



Visual Acuity. — Another argument in favor of the view that the 

 cones are the most important element in vision is presented 'by the 

 close correspondence between the smallest possible image and the 

 disposition of the cones in the fovea. Salzer^ has shown that 0.01 

 mm^. of fovea contains 138 cones, and that an illuminated sheet is per- 

 ceived as such only if each cone is the recipient of at least one ray of 

 light. This necessitates about 140 rays for each 0.01 mm^. of foveal 

 surface. In order to obtain a mosaic impression, the different cones 

 must be invested by a zone of non-stimulated cones. C. DuBois- 

 Reymond^ has estimated the number of rays then required at 72 per 

 0.01 mm^. of foveal surface. Thus, a double star is recognized as two 

 distinct bodies only if the distance between them corresponds to 

 a visual angle of 60 seconds. Quite similarly, two white lines drawn 

 across a black surface, are perceived as two lines only if the distance 

 between them subtends a visual angle of 64 to 73 seconds. At these 

 angles, the image covers an area of 0.0045 to 0.0055 mm^. of retina and 

 involves, therefore, two cones of the fovea. These experiments 

 also show that the visual acuity does not differ greatly within the fovea, 

 and especially not within the foveola. Outside the fovea, however, 

 the acuity diminishes very rapidly, and already at a distance of 20 

 degrees, the Unes of the image must be separated by a distance of 

 0.035 mm. in order to produce separate impressions. 



Guillery has estimated the size of the smallest perceptible image 

 at 0.0035 mm^., this value being apphcable only to the center of the 

 fovea, i.e., to the foveola. For this determination he employed a black 

 dot upon a white background which was gradually moved away from 

 the eye until it just barely disappeared. When thus just barely 

 producing a retinal stimulation, the size of the image may be calculated 

 by correlating its distance from the eyes with its diameter. Inde- 



* Dissertation, Berlin, 1881. 



2 Zeitschr. fiir Psych, und Physiol, der Sinnesorgane, xii, 1896, 243. 



