BELL. — THE PHYSIOLOGICAL BASIS OF ILLUMINATION. 79 



strument rather than on the direct or secondary sensitiveness of the 

 nerve endings to stimulation by light. Great acuity is possibly com- 

 moner among savage peoples than in civilized races. Konig ^ has noted 

 it among the Zulus, whose color vision, by the way, was normal ; it has 

 been found in unusual degree among the Kalmucks, and Johnson ^ noted 

 it in the Congo peoples, in every case associated with slight hj^perme- 

 tropia. Some observations of Johnson (loc. cit.) would suggest that the 

 extremely dark hue of the fundus oculi and consequent diminution of 

 choroidal reflection found among the dark-skinned races may improve 

 the definition, although perhaps at the expense of sensitiveness. It is 

 of course well known that in the last resort the ability to separate 

 objects like neighboring points and lines depends on the minute struc- 

 ture of the retina, and is greatest in the fovea centralis, where the cones 

 are most closely packed. The fovea too is well known to be somewhat 

 less light sensitive than the retina in general. Using a wedge photom- 

 eter, I find for my own eye that there is a difference somewhat exceed- 

 ing one stellar magnitude between the foveal visibility and that outside. 



Following out this line of investigation, it is not difficult to project 

 the fovea as a dull spot in the field of view. Using a wedge photometer 

 and fixing the eye at any point on a large sheet of white paper, one 

 finds, on rather quickly cutting down the light by sliding the wedge, a 

 roundish dark spot exactly in the axis and corresponding in diameter 

 with the projection of the fovea. It is not easy to hold vision of this 

 phenomenon since the axis of the eye inevitably tends to wander. 



By drawing five rather faint crosses at the centre and corners of a 

 square, say a decimeter on a side, one can, by careful manipulation of 

 the wedge, make the central cross disappear in the foveal blind spot 

 while the corner crosses remain visible. The facts regarding the 

 independence of acuity and sensitiveness lend weight to the theory of 

 our confrere Professor Lowell regarding the bearing of this matter on 

 astronomical observations. Extreme acuity and extreme sensitiveness 

 being both rather rare, any considerable degree of independence must 

 render the coexistence of both in the same individual unusual in a very 

 much higher degree. 



The failure of acuity in a dim light is familiar, and its variation with 

 intensity affords an independent criterion of the necessary requirements 

 in artificial illumination. Enough light must be provided to bring the 

 eye to its normal acuity as well as to its normal value of Fechner's 

 fraction. Fortunately the researches of Dr. Uhthoff^ and of Drs. 



1 Nature, 31, 476. 2 Phil. Trans., 194, B. 61. 



3 Graefe's Arch., 32, 171 ; 36, 33. 



