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 
