180 Mr. H. E. Ives on the 



blue by the other method. To explain the low sensibility 

 and low critical speed for peripheral vision — an apparent 

 reversal o£ the effect caused by increasing the size o£ the 

 field — it is only necessary to assume the cones to be present 

 in too small number, as the distance from the centre of the 

 field becomes increasingly large, to play an important part. 

 We then obtain rod flicker, which is produced by low speeds 

 of alternation. 



Along some such lines as this it is possible that the pheno- 

 mena of the flicker photometer may ultimately receive 

 explanation. The present work has not been carried suffi- 

 ciently far into extreme conditions of illumination and 

 localization upon the retina to establish that the action of 

 the flicker photometer is to be ascribed chiefly to the cones, 

 although this may appear probable. Even were this estab- 

 lished there would still remain the larger part of the problem. 

 For how do the cones fuse the colour impressions previous 

 to fusing the "luminosity" impressions? Were the facts 

 exactly opposite, if intensity flicker disappeared first with 

 increased speed and colour flicker persisted, we could ascribe 

 the luminosity phenomena to the rods or organs of colourless 

 vision which possess greater inertia, and the colour pheno- 

 mena to the cones, in accordance with the commonly known 

 properties of each. Unfortunately, we must depend upon the 

 cones for both the colour and flicker photometer phenomena: 

 hence the need for ascribing to them some complexity of 

 function as suggested above, and the difficulty of compre- 

 hending how the separation of luminosity and hue takes place. 



Morris-Airey has suggested that the differences between 

 the flicker and equality of brightness photometers may be 

 ascribed to the different rates of the rise of sensation with 

 different colours, a phenomenon which could affect the flicker 

 but not the other instrument. For instance, in the flicker 

 photometer, when adjusted to show no flicker, if either light 

 source is obscured, the violently flickering illumination from 

 the other source gives a decidedly greater impression of 

 brightness than if the speed is increased until flicker dis- 

 appears, or if one-half the steady radiation from that source 

 is viewed. This apparently increased brightness has been 

 found different for different colours. If this phenomenon is 

 taking place even in the absence of all flicker, when the 

 flicker photometer is at its mid-position, then the different 

 amount of the effect for different colours might cause a 

 difference in the readings of the photometers. 



As yet this theory has not apparently received any thorough 

 experimental test. It is not established that at the same 



