ELEMENTARY EXPERIMENTAL PHYSIOLOGY 249 



grounds On rotating this disc it will be observed that the flicker 

 persists much longer in the outer blue and black ring than in the inner 

 blue and black ring. 



FIG. 218. 



Fechner showed that certain colour effects may be produced by slow 

 rotation of discs which consist of black sectors of increasing size on a 

 white ground. They may also be seen in a disc showing black circular 

 lines of different circumferences on a white semicircular area, the other 

 half of the disc being black. Such a disc is shown in Fig. 218. 

 It has been shown that a bright object on a dark background appears, 

 when suddenly exposed, to be surrounded with a red border lasting a 

 fraction of a second. If the illumination be brighter a blue green effect 

 is visible. These facts in part explain the appearance of colours shown 

 when the discs below are rotated.] 



4. The Fusion of Colour Sensations. Several methods have been 

 devised with the object of enabling us to fuse separate colour sensations. 

 These depend either upon separate colours forming images on the 

 retina in such rapid succession as to be inseparable, or else upon 

 separate colours forming images in the same portion of the retina so 

 that the sensations are super imposed. 



The first method is generally carried out by means of the separate 

 colours being arranged as sectors in a circle, which is rapidly revolved 

 about its centre, the instrument adapted for the purpose being known 

 as a colour-mixer. Discs of different colours, such as the Wundt series, 

 are obtainable, and each disc has a radial slit at one point so that these 

 can be arranged upon a common centre and a circle may be made up of 

 sectors of various discs. It is desirable to have discs of two sizes, one 

 about ten inches across, the other four or five inches. It is to be 

 remembered that these discs are not coloured with pure colours of the 



