PHYSICAL PROPERTIES OF LIGHT 491 



mechanism must now be further elaborated in order to permit of the separate apprecia- 

 tion of at least three different fundamental colours. Two courses are open to us : 

 we may either provide three chemical reactions instead of one, each of which responds 

 to light of one fundamental colour, and may assume that the end organ is able ac- 

 curately to determine the amountjof each of the three breakdown products present ; or 

 we may provide three times the original number of nerves and end organs and place 

 them behind colour filters, similar to those used in three-colour photography. Which- 

 ever method be adopted, we should find we had added considerably to the complexity 

 of the sensitive apparatus. Such an apparatus by itself would form a very inefficient 

 organ of vision because it would record only the average quality of the light which fell 

 on it. Some additional mechanism is required by which the direction from which the 

 light rays come may be inferred. Probably the simplest method would be to place 

 each end organ at the bottom end of a Harrow box, the top end of which is open while 

 the sides are covered with a black material in order to prevent reflections. By arranging 

 these boxes radially in relation to a common centre, the apparatus would be capable of 

 localising the direction of a light source. (This is roughly the arrangement found in 

 the faceted eyes of insects.) (See Fig. 251c.) Although such a visual organ can 

 be astonishingly efficient (one need only mention the case of certain dragon-flies, in 

 which the faceted elements number 12,000 to 17,000), yet there can be no question that 

 the use of some sort of optical system which could produce a focussed image of external 

 objects on the sensitive surface or retina would be better still. The employment for 

 this purpose of a mass of high refractive index with a spherical anterior surface at once 

 suggests itself. Certain complications are however introduced at the same time, 

 namely the necessity of changing the focus or accommodating for images at different 

 distances, and of automatically controlling this mechanism in order that "no mental 

 effort may be required for focussing. In order that such an apparatus may be employed 

 with light of different intensity, it is necessary to be able to control the amount of light 

 allowed to reach the sensitive surface. This could be effected by introducing a semi- 

 opaque screen such as the nictating membrane of the bird ; a better plan would be 

 however to employ an opaque screen with an aperture of adjustable size in it, because, as 

 will be shown later, by this means we can reduce the effects of chromatic and other aber- 

 rations. Since the rays which pass through the centre of the refracting -body, or lens, 

 pass through undeviated and therefore with the least amount of aberration, the best 

 place for the aperture would be immediately opposite the centre of the lens, and for 

 similar reasoning its best shape is found to be circular. The diameter of this aperture 

 must be automatically adjustable, according to the intensity of the illumination falling 

 on the sensitive surface of the eye, in order that its action may be independent of mental 

 effort. We may now conveniently consider for a moment the utility of such an organ 

 of vision to its owner. In the first place he will be able to recognise the presence of 

 objects sending light of different intensity and colour into his organ of vision. Move- 

 ment on their part, relative to himself, will be at once perceived, because of changes 

 in the size and position of the area of the retina which is receiving stimulation. Owing 

 further to the way that their images either intercept, or are intercepted by, the images 

 of other objects near them, he will be able to infer their relative position in space, 

 and the distance at which they are placed from him. This estimate will however be 

 very vague, and therefore the judgment of size will be equally uncertain. In the second 

 place we must assume that the whole of the retina, which we have described, is equally 

 sensitive everywhere, and that further the image formed on it by the lens system is 

 equally sharp throughout. There will thus be a very complicated picture of external 

 objects presented to the consciousness of its owner, and it will be correspondingly difficult 

 for him to concentrate his attention on some particular part of the visual field to the 

 partial exclusion of the rest. His organ of vision therefore requires two further improve- 

 ments, one to increase the efficiency of the appreciation of distance, the other to 

 increase his power of concentration. The first might be obtained by greatly increasing 

 the sensitiveness of the mechanism of accommodation, since the focus is altered according 



