LI. Determination of normal vision, a. The acuteness of 



direct vision, b. The range of accommodation. 



c. The amplitude of convergence. 



a. The acuteness of direct vision. 



/. Appliances Charts printed with Snellen's test type; 

 astigmatic chart; test lenses of following strength: 

 +.50 D., +.75 D., + 1.00 D., +2.00 D., + 3.00 D., 

 .50 D., .75 D., 1.00 D., 2 00 D., 3.00 D., 

 + 1.00 D. cyl., + 2.00 D. cyl., 1.00 D. cyl. 2. D. 

 cyl.; simple test frames, and shade; a photometer; Holm- 

 gren's worsteds. 



2. Preparation. Preparatory to testing normal vision it is 

 necessary to make a few general statements regarding: 

 (1) The numeration of lenses. 



The refractive power of a lens is the reciprocal of its focal 

 distance. The refractive power of a lens whose focal 

 distance is 1 m. is, for example, only one-half as great 

 as that of a lens whose focal distance is 0.5 m. Mon- 

 oyer introduced the term dioptre as a unit in measur- 

 ing lenses. One dioptre (1 D.) represents the 

 refractive power of a lens whose focal distance is 1 

 m.; 2 D. corresponds to ^ m.; 3 D. to ^ m.; 4 D. to 

 ^ m., etc. 0.5 D. represents the refractive power of 

 a lens of 2 m. focal distance; 0.25 D. of 4 m. focal 

 distance, and 0.125 D. of 8 m. focal distance. If the 

 lenses are convex (bi-convex) a plus sign is prefixed 

 to the number, i. e., + 5 D., means a bi-convex lens of 

 5 dioptres refractive power, or \ m. focal distance. 

 While 5 D. means a hi concave lens of \ m. negative 

 focal distance. 



232 



