VISION 193 



upward and then in a downward direction, in each case marking 

 where the pencil becomes invisible. If this be done in several diam- 

 eters an outline of the blind spot is obtained, even little prominences 

 showing retinal vessels being indicated. 



(8) To Calculate the Size of the Blind Spot. Helmholtz gives the fol- 

 lowing formula for this purpose: When / is the distance of the eye 

 from the paper, F the distance of the second nodal point from the 

 retina (usually 15 mm.), d the diameter of the sketch of the blind spot 

 drawn on the paper, and D the corresponding size of the blind spot: 



/ d Fd 



= _ orD = : 



Determine the diameter of the blind spot (D) for each member 

 of the class. 



VIII. PERIMETRY. 



In the foregoing experiments we have dealt exclusively with what 

 is called direct vision i. e., with phenomena involving the formation 

 of a clearly defined image upon the macula lutea. Everyone 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 axis of indirect vision 

 extends in every direction from the visual axis, or to locate the 

 perimeter of the field of indirect vision. Various instruments have 

 been devised, called perimeters, to aid one in perimetry. 



All of these appliances have for their object the mapping 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. 81); 

 perimeter charts, such as shown in Fig. 82. 



2. Preparation. A very economical and accurate perimeter may 

 be constructed in the following manner: 



Take a blackboard whose dimensions are about 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 surface 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 degrees. Reference to Fig. 80 makes it evident that if the line A B 

 represents the plane surface of the blackboard, and if the eye be placed 

 at 0, the equal increments of 10 degrees on the quadrant become a 

 series of increasing increments upon the surface of the board. The 

 numbers at the right (Fig. 80) show just how many centimetres the 

 radius of each successive circle should be, provided the distance of 

 the eye from the board be taken at 20 cm. 



13 



