544 SCIENCE PROGRESS 



vision by very faint light. As Sir William Abney and many 

 other investigators have shown, the maximum (sunlight) lumin- 

 osity for high illuminations commonly occurs near 0*58^; at low 

 illuminations it is nearer 0*53/^; whilst with a fully dark-adapted 

 eye and a very faint illumination indeed, it may even be found 

 near 0*5 1//.. 



There are other marked differences in the behaviour of the 

 eye at high and low illuminations which serve to suggest that 

 vision is accomplished in a radically distinct manner under the 

 two conditions. According to one theory, vision at very low 

 illuminations is accomplished by the aid of minute retinal organs 

 known as the " rods " (which are presumed to be colour-blind), 

 whilst at high illuminations an entirely different set of organs, 

 the "cones," which are invested with the power of perceiving 



o"4 o's o"6 o'7 o'8ja 



Fig. 2. — Showing sensitiveness of eye at high illuminations (curve drawn full) 

 and at low illuminations (curve drawn dotted). 



colour, come into play. The rods are also credited with being 

 practically blind to red light but very sensitive to blue-green 

 light, which, however, is perceived as white. Consequently 

 in the faint twilight red objects, such as geraniums, appear dead 

 black, whilst green grass appears an uncanny grey tint. 



It would carry us too far away from the subject of this 

 article to pursue the discussion of these fascinating peculiarities 

 of the eye, some of which have been proved to have a very 

 direct and even disconcertingly direct bearing on photometry. 

 The phenomena are well established but the explanation seems 

 to be still the subject of dispute among physiologists. For 

 example, Dr. F. W. Edridge Green has expressed disagreement 

 with the view of the functions of the rods given above, pre- 

 ferring a theory of vision based largely on the behaviour of 



