CONTENTS 



XV 



PAGE 



II. The Perception of Colour ......... 619 



Objective Methods of Investigation ...... 621 



Subjective Methods of Investigation ...... 623 



1. The Colour Vision of Cyclostomes ....... 624 



2. The Colour Vision of Fishes 624 



3. The Colour Vision of Amphibians ....... 627 



4. The Colour Vision of Reptiles ....... 628 



6, The Colour Vision of Birds 629 



6. The Colour Vision of Mammals ....... 632 



III. The Perception of Form ......... 637 



1. Optical Factors 638 



(a) The Refraction of Vertebrates ...... 638 



(6) Accommodation in Vertebrates ...... 640 



(i) Static Devices ........ 640 



Stenopoeic Pupil, 641 ; Duplicated Optical System, 

 641 ; Interposition of Nictitating Membrane, 643 ; 

 Duplicated Retina, 643 ; Ramp-retina, 643 ; Cor- 

 rugated Retina, 643 ; Length of Receptor Elements, 

 643 



(ii) DjTiamic Devices ....... 644 



(a) Movement of Lens as a ^^^^ole .... 644 



Backward Movement, 644 ; Forward Movement, 

 647 



(^) Deformation of Lens ...... 649 



By Direct Ciliary Pressure, 649 ; by Capsular 

 Elasticity, 652 



(iii) Accommodation in Amphibious Vertebrates . . . 654 



(c) Other Optical Factors determining Visual Acuity . . . 655 



2. The Structure of the Retina 656 



(a) The Area Centralis ........ 657 



(b) The Fovea 658 



(c) The Degree of Summation ....... 659 



3. The Visual Acuity of Vertebrates ....... 660 



(a) The Visual Acuity of Fishes 660 



(b) The Visual Acuity of Amphibians . . . . . .661 



(c) The Visual Acuity of Reptiles . ...... 661 



(d) The Visual Acuity of Birds 662 



(e) The Visual Acuity of Mammals ...... 663 



rV. The Perception of Space 



1. The Visual Fields of Vertebrates 



(a) The Uniocular Field 

 (6) The Binocular Field 



Cyclostomes, 678 ; Fishes, 678 

 682 ; Birds, 684 ; Mammals, 



2. The Ocular Movements . 



(a) Involuntary Ocular Movements 

 (6) Voluntary Ocular Movements 



Fishes, 693 ; Amphibians, 694 

 Mammals, 696 



Amphibians, 682 

 687 



Reptiles, 



666 

 669 

 669 

 672 



689 

 690 

 692 



Reptiles, 694 ; Birds, 695 



