6i 4 



TEXT-BOOK OF PHYSIOLOGY. 



distance between two points on the retina. It has been experiment- 

 ally determined that the retina can not distinguish two points unless 

 their images are separated by a distance of 0.004 mm. corresponding 

 to a visual angle of 60 seconds. If the distance is less than this the 

 two sensations fuse into one. The reason assigned for this is, that 

 the distance between the centers of two adjoining cones in the 



D=6 D=18 D=60 



FIG. 290. VISUAL ANGLE OF 5 MINUTES. (After Hansell and Sweet.) 



macula is 0.004 mm. With a visual angle of 60 seconds the two 

 foci fall on separate cones; with a smaller visual angle the two 

 foci fall on and excite but a single cone, and hence there arises 

 the sensation of but a single point. 



In ophthalmic practice it is customary in testing the acuteness 

 of vision to employ test type of a certain size for specified distances. 



FIG. 291. THE REFRACTION OF PARALLEL AND DIVERGENT RAYS IN THE EMME- 

 TROPIC EYE IN THE PASSIVE AND IN THE ACTIVE OR ACCOMMODATED CON- 

 DITION. 



Though the entire letter is embraced in an angle of 5 minutes, 

 the strokes are included within an angle of 60 seconds or one minute 

 (Fig. 290). 



Accommodation. In a normal or emmetropic eye, homocentric 

 parallel rays of light (Fig. 291, a, b) after passing through the optic 

 media are converged and brought to a focus on the retina, /. Rays, 



