PROPERTIES OF THE RETINA. 323 



its special functions in vision the fovea centralis possesses a peculiar 

 structure. It forms a shallow depression in the center of the retina 

 described by some authors as elliptical, by others as circular in out- 

 line. In the center of the fovea lies a smaller, very shallow depres- 

 sion spoken of as the foveola. The diameter of the fovea, as stated 

 above, is estimated differently by different authors. While meas- 

 urements on preserved specimens give the diameter as 0.2 to 

 0.4 mm., ophthalmoscopic examination seems to indicate that in 

 the fresh state it may be larger. According to Fritsch,* the fundus, 

 reckoned from the point at which the depression begins, has a diam- 

 eter of 0.5 to 0.75 mm. Within the fovea cones only are present, 

 and these cones are longer and more slender (diameter, 0.002 mm.) 

 than in the rest of the retina. Moreover, the thickness of the retina 

 is much reduced in the fovea, whence arises the depression. At 

 this point the cones are practically exposed directly to the light, 

 whereas in other parts the light must penetrate the other layers 

 before reaching the rods and cones. Lying around the fovea is an 

 area about 6 mm. in diameter, of a yellowish color, and hence 

 known as the macula lutea. According to recent observers f 

 the yellow color of the macula is due to post-mortem changes. 

 In a normal retina this area does not show a yellow color and 

 there is, therefore, no reason why it should be given a special desig- 

 nation. 



Visual Acuity. The distinctness of vision varies greatly in 

 different parts of the retina. It is usually measured by bringing 

 two fine lines closer and closer together until the eye is unable 

 to see them as two distinct objects. Measured in this way it is 

 usually stated that when the distance between the lines subtends an 

 angle of 1 minute (60 seconds) at the eye the limit of visibility is 

 reached. This angle on the retina comprises an area of about 0.004 

 mm. in diameter, 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 distance this object will be invisible, but if brought closer 

 to the eye it will be just seen 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 J 

 estimated the size 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 the size of the physiological point for the 

 fovea varies with the intensity of illumination. The estimates 



243, 1896. 



