Principles underlying the Flicker Photometer. 75 



so used is mainly dependent on the impulses received through 

 the cones. 



The portion of the retina employed in a flicker photometer 

 is usually restricted to the central region, where there are 

 comparatively few rods, and therefore these organs have 

 probably not much to do with its action. Partly on this 

 account, but chiefly because the correct speed for a flicker 

 photometer used to compare widely different heterochromatic 

 lights is so limited, it does not seem likely that informa- 

 tion as to the theory of flicker photometers can readily 

 be obtained by studying the effect of speed on the actual 

 readings of the instrument. It is true that Lauriol * 

 has been quoted as having recorded very marked variations 

 in the reading of a flicker photometer at different speeds ; 

 but on closer examination it appears that this observer's 

 results establish rather the difference between readings of a 

 flicker photometer and a photometer of the equality of 

 brightness pattern. For the difference recorded is that 

 between the readings of a flicker instrument when used as 

 such and when used with the flicker-wheel motionless. 



The writer has sought to determine whether the per- 

 missible range of speed of a flicker instrument used to com- 

 pare moderately heterochromatic sources could be responsible 

 for any marked error, but without detecting any distinct 

 variations due to this effect. 



Coloxir-hlind Observers and the Flicker Photometer. 



Another possible direction in which an inquiry into the 

 theory of the flicker photometer might be pushed is the 

 examination of people having defective colour-vision. Some 

 data are available on this subject, but they lead to somewhat 

 uncertain conclusions. It appears, however, that in some 

 cases colour-blind individuals have been observed to obtain 

 correspondingly abnormal results with both flicker and 

 equality of brightness photometers f, and it would certainly 

 lead one to doubt the soundness of the principle underlying- 

 such instruments were this not the case. 



In reality there seem to be so many different types of 

 colour-blindness, and the existing knowledge of the subject 

 is still so incomplete, that it is difficult for anyone but a 

 physiologist to draw any valid conclusions on the theory of 



* Bull. Soc. Ins. des Elect riciens, 1904. 



t See Brodlnm, Gas World, Feb. 15. 1908 : Dow, Electrician, 

 Feb. 1 & 8, 1907; Tufts, Phys, Review, Dec. 1907, &c. 



