OPTICS 277 



Two separate object points are seen distinctly when their 

 image points fall upon two sensational elements separated 

 by at least one other element. 



Acuteness of vision is the power by which two luminous 

 points can be distinctly seen. The acuteness of vision is the 

 greater the smaller the diameter of the sensational element 

 or the smaller " the smallest visual angle," when the lumi- 

 nous points can still be seen as distinct points. The visual 

 angle is the angle which the direction rays from the luminous 

 points form with each other. 



\Ye can discriminate between direct and indirect vision. 

 A point seen by direct vision is called the fixed object point. 

 We see less distinctly by indirect than by direct vision. 



The fovea centralis functions in direct vision. It has the 

 greatest acuteness of vision, and by means of it we can, under 

 favorable circumstances, distinguish two luminous points 

 which form a visual angle of 40 seconds. This corresponds 

 to a diameter of 3yu of the sensational element (a little more 

 than the diameter of a cone in the fovea). The peripheral 

 portions of the retina function in indirect vision and have less 

 acuteness of vision. 



Oculists regard five minutes as normally the smallest visual 

 angle for the fovea, which is true for the method generally in vogue 

 for determining acuteness of vision (reading of letter). 



The direction ray drawn through the fovea is called the 

 visual axis or line of fixation (see Fig. 28, G G'}\ in it lies 

 the luminous point which we fix upon in seeing. This line 

 does not coincide with the optical axis, but its anterior end 

 is a little inward from the optical axis. The angle formed 

 by these two lines is called the " angle a " and is about 7. 



The visual field is the field in which all the luminous 

 points seen by the stationary eye seem to lie. The visual 

 field therefore includes all the directions in which the station- 

 ary eye can see objects. Its extent is indicated by the 

 angles which the lines drawn from the limits of the visual 

 field through the nodal point form with the visual axis. 

 The extent of the visual field is outward 70-90, inward 



