But what, it may be asked, can be the purpose of an eye-structure which gives, not an image, but merely a spot of light? The answer to this question can only be found when it is remembered that there are thousands of these facets and cones giving thousands of spots of light. The somewhat divergent cones and facets of the insect's eye (Fig. 37) embrace, as a whole, an extended field of vision; each has its special point in that field; and each conveys to the nerve-rod which lies beneath it a stimulation in accordance with the brightness, or intensity, or quality of that special point of the field to which it is directed. The external field of vision is thus reproduced in miniature mosaic at the points of the crystalline cones--thus there is produced by the juxtaposition of contiguous points a stippled image. And it must be remembered that, even in human vision, the stimulation is not that of a continuum, but is stippled with the fine stippling of the ends of the rods and cones. In insect-vision the stippling is far coarser, and the image is produced on different principles. [Illustration: Fig. 38.--Diagram of mosaic vision.] In the vertebrate the image is produced by a lens; in the insect's eye, by the elongated cones. How this is effected will be readily seen with the aid of the diagram. At _a b_ are a number of transparent rods, separated by pigmented material absorbent of light. They represent the crystalline cones. At _c d_ is an arrow placed in front of them; at _e f_ is a screen placed behind them. Rays of light start in all directions from any point, _c_, of the arrow; but of these only that which passes straight down one of the transparent rods reaches the screen. Those which pass obliquely into other rods are absorbed by the pigmented material. Similarly with rays starting from any other point of the arrow. Only those which, in each case, pass straight down one of the rods reach the screen. Thus there is produced a reduced stippled image, _c' d'_, of the arrow. There has been a good deal of discussion as to the relative functions of the ocelli and the facetted eyes of insects. The view generally held is that the ocelli are specially useful in dark places and for near vision; while the facetted eyes are for more distant sight and for the ascertainment of space-relations. How the two sets of impressions are correlated and co-ordinated in insect-consciousness, who can say?[FJ]