Contributions to the Study of " Flicker" 351 



the stimulus given to the retina lasts undiminished a shorter time 

 for the yellow than for any other colour, but they give the very 

 approximately exact relative " last " for the different colours. The 

 general form of the curves following very closely the curve which 

 expresses the luminous intensity of the spectrum obtained by 

 Newton, Abney, and others. 



The next point seems to the writer very important, especially with 

 reference to what follows. Numerous experiments were made to find 

 out how the "last" (undiminished) for any one colour varied with 

 the intensity of the light ; for the present it is sufficient to say that 

 in every case the more intense the illumination the more rapid must 

 the rotation of the disc be made before nicker will vanish* Hence 

 we are bound to infer that as the stimulus applied to the retina 

 increases in intensity, the impression produced retains its maximum 

 value for a shorter and shorter time. 



That a brighter illumination does produce a greater stimulus (i.e., 

 that neither the contraction of the pupil, nor any other cause, over- 

 comes the effect of brighter illumination) is conclusively proved by 

 the fact that the brighter the light the brighter on the whole is the 

 disc when nicker has just vanished. 



Research was next made to find out in what way the rotation of 

 a disc must be varied for flicker to vanish when the proportion of 

 the coloured to the black sector varied, the intensity of the illumina- 

 tion remaining constant. 



Two discs were taken : one painted with Indian ink laid on as dry 

 as possible, and the other of the whitest cardboard procurable ; these 

 were dovetailed after Maxwell's method and mounted on a motor 

 with a tk syren " disc. This last was pierced with six, twelve, or 

 twenty-four holes, that the note emitted might lie within the octaves 

 which are both easy to sing and in which intervals can be most 

 correctly judged. 



As the experiments took many days and nights, and it was most 

 important that the conditions should be as similar as possible 011 

 different occasions, the whole of the apparatus remained unmoved 

 throughout the whole set of comparative experiments ; the lime-light 

 burner was firmly screwed down to the base-board of the lantern. 

 The rotating disc was most carefully screened from extraneous light, 

 though if the writer repeated the experiments he would wear a black 

 mask to prevent the illumination of the dark sector of the disc by 

 even the faint light reflected from the observer's face. The writer 

 thinks this may account partly, though not wholly, for the asym- 

 metry of the curves of fig. 3 in the case of the red and yellow. The 

 grating was surrounded by a screen coated with the dullest optical 

 black, whilst another screen allowed only the special colour required 

 to fall on. the half of the disc viewed. Each observer regarded the 



