Photometry of Lights of Different Colours. 367 



they show the same distribution of brightness in the spectrum. 

 The capacity of different methods to summate will form the 

 subject of a subsequent paper. 



Peripheral versus Central Vision. 



The greater sensibility of the peripheral retina to motion 

 and to flicker has been frequently observed. In the work 

 on the flicker photometer the possibility of using this greater 

 peripheral sensitiveness was investigated, with the unexpected 

 result of finding a smaller sensitiveness than for central or 

 foveal vision. During the course of the present work 

 frequent comparisons were made between central and peri- 

 pheral sensitiveness to flicker, with different colours, and 

 with large and small fields, at different illuminations. The 

 findings explain satisfactorily the failure of peripheral vision 

 with the flicker photometer. 



The most significant fact was learned on the failure of 

 several attempts to secure the critical frequency-illumination 

 relationship by peripheral vision settings, namely, that such 

 greater sensitiveness as the periphery possesses quickly 

 disappears on continued gazing. It is, at least at medium 

 and at high illuminations, merely a momentary phenomenon, 

 not at all available for careful and continued photometric 

 settings. The periphery, by the fatigue due to steady use, 

 becomes for the long wave end of the spectrum, and at high 

 illuminations, less sensitive than the centre of the retina. 

 The fovea alone appears to have properties sufficiently 

 constant and dependable for photometric use. 



After this fact was established qualitative observations 

 were carried out, with the eye thoroughly rested between 

 observations. The critical speed for central vision was 

 established, the eye was directed so that the image fell about 

 7° from the fovea, usually on the nasal side, and the direction 

 of change of speed necessary to produce or destroy flicker 

 was noted. These observations showed that for very high, 

 nearly dazzling illumination, there is little choice between 

 centre and periphery with red and blue light, with large or 

 small field. A large retinal field is always more sensitive 

 than a small one, apparently a pure area effect. 



At lower illuminations the periphery becomes more and 

 more sensitive to blue flicker, but less to red flicker. At 

 very low illuminations (about 12 cycles per second) with a 

 small field the difference between the behaviour of the two 

 colours is enormous. By direct vision a blue field is almost 

 invisible and flicker is perceived only at a low speed ; by 



