Photometry of Lights of Different Colours. 181 



illumination the rates of rise of sensation are actually different 

 for different colours, and phenomena of this sort are extremely- 

 apt to be influenced by the order of the illumination. Nor 

 is it established whether these phenomena are functions of 

 speed rather than of the presence of noticeable flicker. In 

 other words, whether the disappearance of flicker by alter- 

 nation with another colour does not destroy these effects as 

 completely as does the disappearance due to higher speed 

 with the one colour. There is much yet to be investigated 

 here, and in view of the differences between observers in 

 their results as to the differences between the two methods, 

 it is clear that these phenomena of rise of sensation must be 

 studied with the same eye and apparatus by which are made 

 the flicker measurements in which they may play a part. 



And so it appears that there is no theory of the flicker 

 photometer sufficiently supported by known phenomena to 

 be at present satisfactory. The data collected in this inves- 

 tigation are probably insufficient to throw much light on the 

 actual processes at work in the retina, and the investigation 

 up to the present has had for chief object, apart from the 

 practical photometric information so urgently needed, the 

 location of landmarks from which to start the more searching 

 investigations into the nature of the flicker phenomena and 

 the brightness sensation. 



Investigations which are now being carried on, or are 

 planned with the same apparatus, are expected to furnish 

 data which will assist in the formation of satisfactory theories. 

 Prominent among these investigations may be mentioned : 

 Luminosity curves as given by the method of critical fre- 

 quency ; the comparison of the physical and arithmetical 

 summation of measured illuminations; the effect of changing 

 the hue of the comparison source ; the phenomena of rise of 

 sensation as they occur in the flicker photometer as used ; 

 the connexion between hue difference, speed, and sensibility 

 in the flicker photometer. 



Bearing upon Practical Photometry. 



The primary object of this investigation is to decide upon 

 the photometric method and conditions to hold in deter- 

 mining the candle-power values of lights of different colours. 

 Until the completion of the work laid down the full specifi- 

 cation of these conditions cannot be made. There is as yet 

 no answer to the question — What is the candle-power of a 

 light in terms of a standard of different colour ? Certain 

 conclusions may, however, be drawn at the present stage of 

 the work which will hold no matter what the final answer to 



