Principles underlying the Flicker Photometer. 61 



a somewhat abrupt alteration in the nature of colour-vision 

 occurs, in the case o£ the author's eye, near 02 lux. 



The instrument was in this case a Joly photometer, the 

 blocks of which were illuminated by red and green right 

 furnished by two carbon-filament glow-lamps screened with 

 ruby-red and signal-green glasses respectively. It will be 

 seen that, as the eye is withdrawn, the reading invariably 

 tends to favour the red light at the expense of the green. 

 This, according to the rod and cone theory, is to be explained 

 on the supposition that, as the image falls more and more 

 towards the centre of the retina, the number of rods, as com- 

 pared with cones, in the area covered by it, continually 

 decreases ; eventually, when the fovea centralis is readied, 

 a more or less steady state of things should be arrived at 

 since in this region of the retina only cones are believed to 

 exist. 



For convenience in comparing the steepness of the curves 

 at different illuminations, all the readings are reduced to a 

 scale such that the value corresponding with the eye at a 

 distance of 10 centimetres is unity. It will be observed that, 

 as we should expect, we obtain in each case a curve which 

 gradually rises but afterwards tends to become horizontal. 

 But it will once more be seen how very much steeper the 

 curve suddenly becomes when the illumination is reduced to 

 the order of 0*2 lux (metre-candles). On the other hand, an 

 increase in the intensity of illumination from 10 to 40 lux 

 has relatively little effect. All this is exactly what 

 the theory would lead us to expect. It is apparently only at 

 comparatively low illuminations that the struggle of the rods 

 and cones becomes marked. And at an order of illumination 

 about 0*2 lux (metre-enndles) the cones might be supposed 

 abruptly to cease exercising their powers, with the result that 

 the relatively small number of rods in play become propor- 

 tionally more effective. A small change in the retinal con- 

 ditions would therefore be much more influential in disturbing 

 the judgment of! balance than at high illuminations, and the 

 curve becomes correspondingly steeper. 



The above experiment, therefore, is interesting not only 

 because of the confirmation it seems to afford to the provisional 

 rod and cone theory, but also because it serves to bear out the 

 suggestion that an abrupt change in the retinal conditions 

 (seemingly adequately explained by the supposition that the 

 cones go out of action or at any rate suddenly decrease 

 in sensibility) occurs at an illumination near 0"2 lux. 

 This figure will subsequently be recalled in connexion with 

 experiments on the flicker photometer. 





