VISION. 233 



The use of cylindrical lenses is frequently necessary. 

 A cylindrical lens is a section of a cylinder parallel to 

 its axis. Cylindrical lenses may be convex or concave. 

 A convex cylindrical lens capable of bringing rays to a 

 linear focus at a distance of one half meter would be 

 designated as follows: -f- 2 D. cyl. 

 (2) Test types and visual angle. 



The visual angle is that included between lines joining 

 the extremities of an object and the nodal point, or the 

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

 Fig. 29 the object at d subtends the angle v, while 

 the object at D 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^distance of object 

 from nodal point, n = distance of image from nodal 

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



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



(2) o = 1 Ld; 



= tan. 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 dis- 

 cernible to the average normal eye at that distance? 



At 1 m. height of letter, o = 0.001454X 1000 = 1.45 

 mm. 



Determine the height of the letters for each of the 

 following distances 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 

 readilv in the emmetropic eye bring up its definition 



