THE THEORY OF ARTHROPOD VISION. 571 



trast, on the other hand, in insects which are only abroad 

 in the sunlight, as these have long, slender great rods, which 

 must transmit far more slender pencils of light. Many of 

 the experiments of Plateau and others are vitiated by the 

 fact that they have tried experiments on such insects in diffused 

 daylight instead of in brilliant sunlight, which in many insects 

 appears to be indispensable for acute vision. 



I have given some calculations of the intensity of the 

 illumination of the retinal image in insects (p. 575), but the 

 subject is one which has hitherto been neglected, and further 

 experiments are needed. All small birds which are diurnal 

 in their habits are apparently quite incapable of vision in a 

 feeble light, and are readily tamed in the dusk ; the absolute 

 helplessness of diurnal insects in the dusk has already been 

 remarked upon by Forel and others, and indicates that a 

 powerful light is generally necessary for distinct vision in such 

 creatures. 



Movements of Objects in the Visual Field. Both Plateau and 

 Exner believe that the movement of objects in the visual field 

 renders their perception more easy, although I confess I do not 

 appreciate the manner in which movement can render an im- 

 perfect picture more perfect. It seems to me that, by adding 

 a new condition, this hypothesis merely makes the compre- 

 hension of the manner in which vision is effected more obscure. 

 On the principle by which many accept a mathematical 

 formula they do not understand as a proof, some will un- 

 doubtedly accept a complex theory beyond their compre- 

 hension as good, rather than one which they are capable of 

 testing and in which they are able to detect errors. The 

 movements of objects in the visual field undoubtedly affect 

 visual judgments as to distance, and I have no doubt that in 

 some way insects do know the distance of the objects amongst 

 which they are moving. As, however, we do not know how 

 we determine the distance of objects unless they are seen by 

 binocular vision, it would be useless to discuss the manner in 

 which insects determine distance. 



Binocular Vision. In some parts of the visual field of insects 



