372 PRACTICAL PHYSIOLOGY 



The distinctiveness of the colours is less marked than in the projected 

 spectrum examined on the screen, and the different sensations all 

 approach that of white. The red, however, does not pass beyond a 

 yellow sensation. With much diminished intensity colours become less 

 recognisable. Red in particular is difficult to recognise in a much 

 diminished light. 



EXPERIMENT II. Take a small square of red paper and a similar 

 piece of blue paper which in a light of moderate brightness appear of 

 approximately equal intensity. Carry these to an almost dark room 

 and note the dulness or even blackness of the red whilst the blue may 

 still appear bright. 



EXPERIMENT III. A similar experiment to Experiment II. may be 

 performed with the material of Experiment I., Section H, of the Milton 

 Bradley Pseudoptics. Taking the red and blue discs it will be noticed that 

 the red is much the brighter colour in ordinary light. But if the two 

 cards be carried to an almost dark room, it will be possible to dis- 

 tinguish the blue disc far more easily than the red. 



3. The Fusion of Distinct Sensations of Black and White. Flicker. 

 This fusion depends upon the persistence of the positive after-images 

 each separate stimulus brings about. If separate stimuli follow each 

 other sufficiently rapidly a blending of the different sensations occurs, 

 as is well exemplified in the presentation of the series of rapidly 

 succeeding views in the cinematograph. The phenomena upon which 

 this depends can be shown in a revolving disc divided into rings of 

 sectors of white and black, increasing in number from the centre to the 

 circumference. Such a disc is included in the Petzold series. 



EXPERIMENT I. Rotate a disc such as (Fig. 243) slowly, and note 

 that at a certain rate the peripheral ring appears as a uniform grey, a 

 flickering sensation is produced on the neighbouring rings, but the 

 central rings show an alternation of white and black. Increase the rate 

 and note that these can also be caused to blend. 



In general it may be stated that when fusion in any way occurs the 

 resulting sensation of grey is the same as if the light reflected inter- 

 mittently were replaced by the same quantity of light continuously 

 reflected, in other words, as if a uniform grey of a certain shade were 

 substituted for the series of sectors ; moreover, if the rate at which the 

 sectors are successively presented to the retina be increased above that 

 necessary for fusion, the intensity of the resulting sensation is not 

 altered. (Talbot-Plateau Law). This may be experimentally confirmed 

 in the following manner. 



EXPERIMENT II. Take a lens of short focus and hold it at such 



