344 



THE SPECIAL SENSES. 



angle on the retina comprises an area of about 0.004 mm. in diam- 

 eter, sufficient to cover two cones in the fovea. A simpler method 

 to ascertain the size of a just perceptible image on the retina is to 

 use a black spot upon a white background. At a sufficient dis- 

 tance this object will be invisible, the white margins separated 

 by the diameter of the black spot fuse together, but if brought 

 closer to the eye, the spot will be just distinguishable at a certain 

 distance. The diameter of the spot being known, and its distance 

 from the eye, the size of the retinal image may be calculated. 

 Using this method, Guillery* estimated the diameter of the just 

 perceptible retinal image, or, as it has been appropriately called, 

 the physiological point, at 0.0035 mm. These estimates apply only 

 to the fovea, and, indeed, to the central part of the fovea, the 

 foveola. Numerous authors have called attention to the fact that 



&* 30 



10*3' O 9 



Fig. 145. Curve to show the relative acuity of vision in the central and peripheral fields 

 and in the light-adapted and the dark-adapted eye. (Koester.) The full line represents 

 the relative acuteness of vision in the eye exposed to usual illumination. From the center of 

 the fovea, 0, the acuity of vision falls rapidly at first and then more slowly as one passes out- 

 ward into the peripheral field. The dotted line represents the acuity of vision in dim lights. 

 The fovea, under this latter condition, is less sensitive than the parts of the retina at an angular 

 distance of 10 or even 69. 



visual acuity, as measured by the least distance at which two ob- 

 jects may be seen separately, varies with the intensity of illumina- 



* Guillery, "Zeitschrift f. Psychologie u. Physiol. d. Sinnesorgane," 12, 

 243, 1896. 



