Contributions to the Study of " Flicker'." 349 



that the writer's eyes are perfectly normal, and that he has a good 

 ear. There are so many experimental details, that to describe fully 

 a single observation, and to state the reasons for the many precau- 

 tions which mast be taken if the result is to be of any value, would 

 take up so much space that the writer thinks it best only to mention 

 a few of the more important : The same spot on the lime was never 

 used twice ; the width of the slit was kept unaltered ; each disap- 

 pearance of nicker was witnessed by two, sometimes three, observers ; 

 the same time as nearly as possible was maintained between the 

 observations, and used in making them, in order that the retina as 

 well as the lime might be in approximately the same condition for 

 each ; the room was completely dark* and the eyes rested between the 

 observations, never looking at any bright objects, and, above all, not 

 at the lime ; the oxygen and coal gas were conveyed by special metal 

 pipes from gasometers in which they were stored under constant 

 pressure outside the building, but since the composition of both 

 gases is liable to variation, the intensity o? the light was maintained 

 constant, by regulating the supply of the gases till the nicker of a 

 half and half disc in the yellow vanished at the same speed of rota- 

 tion (generally A'). After every set of experiments, to make sure 

 that the illumination had not sensibly changed, the note for vanish- 

 ing nicker in the yellow was again observed. Experience teaches 

 that the blue-green is rather better than the yellow for this purpose, 

 and accordingly it was used in later experiments. Not more than 

 two curves, often only one, were drawn in twenty-four hours, and to 

 check the effects of contrast in consecutive experiments in different 

 colours, a curve was drawn (a) by taking the colours of the spectrum 

 from red to violet and vice versa in their natural order, and (6) by 

 finding the pairs of colours on either side of the yellow for which 

 nicker vanished at the same rate of rotation. The results of 

 experiments made thus agree very closely, and the writer may 

 say here that throughout the eight years during which the research 

 has been carried on, the feeling has steadily grown that in sub- 

 jective vision we have to do with immutable quantities, capable of 

 exact measurement, and which vary but slightly, if at all, with 

 different people. 



A glance at the curves in fig. 1 will show that there is a very 

 great difference in the rates of rotation necessary for the disappear- 

 ance of nicker in the different parts of the spectrum, the speed for 

 the yellow being very nearly double that for the violet. 



Also, that on each side of the yellow the rate decreases, until the 

 flicker disappears for the last distinctly visible red at the same rate 

 at which it vanishes for the full green. 



Fig. 2 presents this last fact iu a somewhat different way, and, if 

 * So far as external light is concerned. 



