62 Mr. J. S. Dow on the Physiological 



Meantime it may be pointed out that other experiments of 

 the author* suggest that there is a sudden diminution in 

 visual acuity at just about the above order of illumination. 

 This, therefore, is once more additional evidence that a 

 profound physiological change in the retina occurs near this 

 point. 



Let us now turn to the main question to be considered in 

 this paper, namely, the bearing of the rod and cone theory and 

 the physiological phenomena explained by its aid, on the 

 flicker photometer. Whatever be the exact explanation of the 

 functions of the rods and cones and the retina adopted (and 

 physiolog sts seem as yet to differ considerably in their views 

 on this matter), the physiological effects on which the theory 

 is founded seem to be well authenticated. It is therefore of 

 interest to inquire how far they can be considered applicable 

 to the flicker photometer, which is now beginning to receive 

 a considerable amount of attention as a means of comparing 

 sources of light which differ in colour. As an illustration 

 of the possibility of such instruments yielding results which 

 differ from those obtained by the aid of photometers of the 

 " Equality of Brightness " or " Contrast " types, an experi- 

 ence recently related by Mr. L. Wild may be mentioned. 

 This observer finds that, when an incandescent lamp, having 

 a tungsten filament and run at a consumption of 1*5 watts 

 per G.P., i? compared with a carbon-filament lamp running 

 at 4 watts per candle, the result varies by 6% according as 

 a nicker photometer or an instrument of the ordinary variety 

 is used for the test. Apparently the tungsten lamp has a 

 lower candle-power in the former case f . 



This matter was touched upon in the writer's previous 

 communication to the Physical Society on Colour-photo- 

 metry t. On that occasion it was found that the readings 

 of an Everett-Edgcumbe flicker instrument, when used to 

 compare red and green lights, did not differ, when the distance 

 of the eye from the field of view of the instrument was altered, 

 nearly so greatly as was found to be the case for an " Equality 

 of Brightness 9J instrument. This suggested, therefore, that 

 the behavour of the retina was in some way different, when 

 judging disappearance of flicker, from that characteristic of it 

 when judging brightness. 



It seemed, however, preferable to confirm this result by an 

 experiment in which exactly the same portion of the retina 



* ' Illuminating Engineer ' (London), vol. ii. April 1909, p. 237. 

 t Electrician, July 16, 1909. 

 % Phil. Mag. vol. xx. 1906. 



