196 SPECIAL PHYSIOLOGY 



to locate the twelve or more points located upon the observation chart 

 and join these points into a continuous perimeter. 



(5) In the above experiment we have determined the perimeter for 

 light sensation only; the subject being conscious simply of a light or 

 white spot on a dark ground, but not certain whether the spot is 

 circular or square. 



(6) Determine and chart various color perimeters: (a) red, (6) 

 green, and (c) blue. 



Have the color perimeters the same general form as the white 

 perimeters? If not, describe any noticeable variations. Which of the 

 color perimeters encloses the greatest area ? Enumerate them in order 

 of area. Is this the order which one would expect? Give grounds 

 for position. 



(7) Take corresponding perimeter for the other eye* To use the same 

 blackboard it will be necessary to turn it the other edge up. In what 

 general respect do the right perimeters differ from those of the left? 



(8) With the help of light perimeters of the right and left eyes, 

 determine the field of binocular vision. This is the field of binocular 

 indirect vision. 



IX. DETERMINATION OF NORMAL VISION. 



The Acuteness of Direct Vision. 



1. Appliances. Charts printed with Snellen's test type, astigmatic 

 chart, and test lenses of following strength: + 0.50 D., + 0.75 D., 

 + 1.00 D., + 2.00 D., + 3.00 D., 0.50 D., 0.75 D., 1.00 D., 

 2.00 D., 3.00 D., +1.00 D. cyl., +2.00 D. cyl., 1.00 D. cyl., 

 2.00 D. cyl.; simple test frames and shade; Holmgren's worsteds. 



2. Preparation. Preparatory to testing normal vision it is neces- 

 sary to make a few general statements. 



(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. Monoyer introduced 

 the term dioptre as a unit in measuring lenses. One dioptre (1 D.) 

 represents the refractive power of a lens* whose focal distance is 1 m. ; 

 2 D. corresponds to J m.; 3 D. to J 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 (biconvex) a plus sign is prefixed to the 

 number i. e., +5 D. means a biconvex lens of 5 dioptres refractive 

 power, or ^ m. focal distance. While 5 D. means a biconcave lens 

 of ^ m. negative focal distance. 



The use of the cylindrical lenses is frequently necessary. A 

 cylindrical lens is a section of a cylinder parallel to its axis. 



