L. Perimetry. 



In the foregoing experiments we have dealt exclusively 

 with what is called direct vision, i. e., with phenomena in- 

 volving the formation of a clearly defined image upon the 

 macula lutea. Every one has noticed that outside the range 

 of direct vision one may still get a pretty definite idea not 

 only of form but of color as well. It is the purpose here 

 to ascertain just how far this field of indirect vision extends 

 in every direction from the visual axis; or, to locate the peri 

 meter of the field of indirect vision. Various instruments 

 have been devised called perimeters to aid one in peri- 

 me try. 



All of these appliances have for their object the map- 

 ping of the field. In all exact methods the map takes the 

 form of a polar map, the pole corresponding to the point 

 where the line of vision would pierce perpendicularly the 

 plane of the map. 



1. Appliances. A perimeter, or ruled blackboard, Fig. 32; 

 perimeter charts, such as shown in Fig. 33. 



2. Preparation. A very economical and exact perimeter 

 may be constructed in the following manner : 



Take a blackboard whose dimensions are abont 1 m. by 

 1.5 m. Locate a point 40 cm. from one end and 50 cm. from 

 either side. Let this be the point of fixation or the 

 point where the line of direct vision falls upon the sur- 

 face of the board. 



We propose now to draw upon the board a series of 

 circles whose distance from one another shall represent 

 an angular distance of 10. Reference to Fig. 31 makes 



226 



