1896-97. ] THE FUNCTION OF INDIRECT VISION. 307 



added from the other eye. Further, the function of Binocular vision is 

 not especially to increase the vision field ; it is rather for the purpose of 

 seeing objects from two different standpoints. 



The principle of the qualitative division of labour is, however, directly 

 demanded by the general constitution of consciousness. The facts of 

 consciousness at any time are not all equally prominent, for while all 

 sensations present are perceived, only a very limited number are 

 apperceived, that is, but few are in the "gravitation point" of our 

 attention. Since sensations of sight play the foremost role in the 

 presentational side of mental life it is almost imperative that the 

 manifoldness of light impressions given at any moment should conform 

 to this general constitution. We might, therefore, assume a priori that 

 a highly developed visual organ cannot have an arrangement of its 

 elements like the facetted eyes of the insects (which are a striking case 

 of quantitative division of labour), but rather an arrangement in which 

 each element is to perform its functions in a way different from the 

 other elements. We may then expect that the central part of the retina 

 will be superior to the peripheral regions in some respects and inferior 

 in others. 



Our sense of sight has to apprehend brightness, (light intensity), 

 colour quality, space configuration (shape and size), and changes of the 

 latter, i.e., movement. It is well known that eccentric is far inferior to 

 central vision in the distinction of discrete points and the perception of 

 spacial forms, and also it is known that the manifoldness of colour 

 qualities gradually decreases with the approach to the periphery until a 

 zone is reached in which no colours are perceived at all. It must be 

 noticed, on the other hand, that in the detection of movement, indirect 

 is far superior to direct vision. This is the reason that moving objects 

 so easily draw our attention, even when the objects are small and the 

 movement slow. The most striking case of the superiority of indirect 

 vision, however, is found in its greater sensitiveness to light intensity. 

 In this case indirect vision is physically at a certain disadvantage as 

 compared with direct vision. If wc have a number of lights of equal 

 brightness at equal distance from the eye, but in different parts of the 

 vision field, the retina images of them will not be of equal brightness, 

 but will be brighter the nearer they are to the fovea centralis, for the 

 light which becomes effective in each case is a pencil of rays whose 

 angular value is measured by the projection of the pupil on the plane 

 which is normal to the incidence, in other words the intensity of the 

 retinal image is proportional to the cosine of the angle of incidence. The 

 more oblicjue these cones stand in relation to the pupil the less light 



