VISION 197 



Cylindrical lenses may be convex or concave. A convex cylindrical 

 lens capable of bringing rays to a linear focus at a distance of J m. 

 would be designated as follows: 2 D. cyl. 



(2) Test Types and Visual Angle. The visual angle is that in- 

 included between lines joining the extremities of an object and the 

 nodal point, or the angle subtended by an object at the nodal point. 



FIG. 83 



Illustrating the visual angle (v) and the relation of the distance (d) to the length of the object (o) 

 and image (i). N, the nodal point ; n, the focal distance, the image being on the retina. 



In Fig. 83 the object at o subtends the angle v, while the object at 

 0, though much larger, subtends the same angle v. Now it has been 

 determined by Snellen that the normal eye distinguishes letters 

 subtended by an angle of 5 minutes. If we let d equal distance of 

 object from nodal point, n equals distance of image from nodal 

 point, i length of image, and o of object, then 



(1) i : o : : n : d; 



(2) 



/ox i smev 



(3) but = = tan. v ; 



n cos. v 



(4) o = d tan. v. 



The tangent of 5' = 0.001454; assume d=l m. (1000 mm.), what 

 is the height of the smallest letter discernible to the average normal 

 eye at that distance? 



At 1 m. height of letter, = 0.00145 + 1000=1.45 mm. 



Determine the height for each of the following, respectively: 60 m., 

 30 m., 20 m., 15 m., 12 m., 9 m., 6 m., 4.5 m., 3 m., 2.5 m., 2 m., 

 1.5 m., 1 m., 0.75 m., 0.50 m. 



What is the size of the image in all these cases? A cultivation of 

 the visual power of the eye may readily in the emmetropic eye bring 

 up its definition to J above the average or so that the minimum 

 visual angle for acute vision equals 4'. 



3. Experiments and Observations. (I) To Test the Form Sense. 

 In all of the tests here described it is understood, unless otherwise 

 stated, that the subject sit directly facing the chart, which should be 

 6 m. distant and well illuminated. 



