LXXIV.] 



CONTRAST. 



357 



279) may be used. Look vertically through the glass at the horizontal white 

 paper, and observe a pale red tint. Attach a small black square to the centre of 

 the vertical arm at B, the image of this square is seen at b as a deep red image. 

 Place a similar black square on the horizontal board at C, it should appear 

 grey ; but a grey on a red ground causes 

 contrast, and so one sees a greenish-blue 

 square alongside a red one. 



6. Bering's Apparatus for Simultaneous 

 Contrast. 



(a. ) By means of one or two doubly 

 refractive prisms (fig. 280, P, P) a double 

 image is obtained of narrow strips of 

 coloured paper placed either on a white or 

 a coloured background. If blue be placed 

 on yellow, the double image is bluish, and 

 if yellow be placed on blue, the double 

 image is yellowish (PJiuger^s Archiv, vol. 



47, P- 237)- 



(b.) The apparatus of Hering (fig. 281) 

 is also useful for simultaneous contrast. 

 Coloured glasses (e.g., blue and red) are 

 placed in P and P', and light is reflected 

 through them by the adjustable white 

 surfaces (W, W). On looking at a narrow 

 black strip (8) on a white ground, one sees 

 contrast phenomena according to the colours of the glass used. 



The white surface in front of the red glass, when looked at with one eye, is 

 red, just as that in front of the blue glass is blue under the same conditions. 

 Focus the eyes for an object nearer than the black strip on the white ground. 



FIG. 279. Bering's Apparatus for 

 Contrast. 



FIG. 280. Hering's Apparatus for Simultaneous 

 Contrast with Binocular Vision by two 

 Doub y Refractive Prisms, P, P. G. Glass 

 to avoid reflection. 



FIG. 281. Hering's Apparatus for Simultaneous Con- 

 trast. P, P'. Coloured glasses; W, W. White 

 reflectors ; S. Black line on white surface. 



This is done by looking at a bead (k) fixed on the point of a rod (supplied 

 with the instrument), the latter being held between the eyes and the white 

 ground. The black strip seen under these conditions forms a double image, 

 i. e. its image is formed on two non-corresponding parts of the retina. The 



