76 Physiological Principles underlying Flicker Photometer. 



the flicker photometer by an examination of colour-blind 

 individuals. The study of the principles underlying this 

 instrument must, it would seem, be conducted in the light of 

 the modern theories of the action of the retina. Physiologists, 

 however, seem to have found some difficulty in reconciling 

 many forms of colour-blindness with the usual theory of the 

 action of the rods and cones. As an illustration of the com- 

 plexities of the subject it may be mentioned that according 

 to Edridge Green * some forms of colour-blindness arise, 

 not through imperfect retinal apparatus, but through con- 

 fusion at the centres of perception of colour in the brain ; 

 the same observer states that colour-blindness frequently 

 accompanies some forms of insanity. 



Concluding Pemarks. 



In conclusion the writer would like to say that he is 

 conscious that the data presented are in many respects 

 incomplete, and that other and more adequate experiments 

 are needed before any decision as to the correctness of the 

 suggestions brought forward can be taken. It maybe noted, 

 for instance, that these results described were, like those 

 described in previous papers, obtained for the author's eye 

 only. For this reason they are strictly comparable with 

 the latter and in some respects seem very consistent. But 

 naturally in order to be conclusive experiments in photometry 

 should be based upon the observations of a large number 

 of individuals, and upon a number of different types of 

 instruments. 



These experiments were mainly carried out at the Central 

 Technical College, South Kensington, during 1906 and 1907, 

 and the author had hoped to make the series of researches 

 described in this paper somewhat more complete. Circum- 

 stances having prevented this, however, it occurred to him 

 that their present publication might lead others to investigate 

 the phenomena described more closely from the standpoint of 

 photometry; the need for gaining some insight into the 

 physiological facts underlying the flicker photometer may, 

 perhaps, give these experiments a certain suggestive value. 



The writer is also well aware that the problem investigated 

 is so essentially physiological in character as to be strictly 

 only capable of beiDg adequately studied by the combined 

 efforts of the physiologist and the physicist interested in the 

 problem from the photometrical side. It may be stated, too, 

 that many points in connexion with the theory of the action 



* 'Colour-Blindness and Colour -Perception.' 



