THE PERCEPTION OF SPACE 



677 



visual axes exist and are nearly central in location, such measurements are of 

 more value ; for this reason Lindsay Johnson's (1901) extensive observations 

 on Mammals give a good indication of the binocularity within this class. ^ 



The most efficient and reliable method yet evolved for the determination 



Most Fishes and 

 Lagomorpha 80-85° 



Giraffe 75° 



Dog 25° 



Lemur 15° 



Cat 13' 



Fig. 813. — The Angle Gamjl^ in the Vertebrate Phylum. 



of the binocular field is that which depends on clamping the dissected head of 

 the animal in the central position on a perimeter, moving a light along the 

 arc and observing its image as seen through the sclera; on moving the 

 light in all directions the extent of the field within which the image falls on the 

 retinse of both eyes simultaneously can be plotted out (Fig. 814). In the hands 

 of Tschermak (1902), Rochon-Duvigneaud (1921-23), Verrier (1930), Kahmann 

 (1932) and Pisa (1939) this technique has given sat ivsfactory results. 



1 p. 688, Fig. 837. 



