362 PRACTICAL PHYSIOLOGY. [LXXIV. 



hence the red is regarded as nearer. This is not a sufficient explanation, as 

 many see the blue nearer than the red. The apparent difference disappears 

 on closure of one eye, but on opening the other eye, the difference of distance 

 asserts itself. Is this due to stereoscopy? Einthoven supposes that it is. 

 (Einthoven, "On the Production of Shadow and Perspective Effects by Difference 

 of Colour," Brain, 1893, p. 191.) 



(b.) Briicke showed that the retinal images of differently coloured points 

 are shifted with respect to one another. Fix on a black background a narrow 

 vertical strip of paper, the upper and lower thirds being red and the middle 

 third blue. On looking at the strip with one eye the blue part deviates to 

 one side and the red to the other side. " By covering either eye alternately 

 a deviation of the red and blue parts in opposite directions will be observed ; 

 and on both eyes being used, the notion of a difference in distance is proved 

 by the combination of the two images in such a way that the parts that 

 deviate to the nasal side constitute the nearer image, the parts that deviate 

 to the temporal side, the further image. " Einthoven finds that the stereo- 

 scopic effect is more marked with the coloured letters. 



(c.) The relative removal of the differently coloured images is due to the 

 excentricity of the pupil. The pupils may be made highly excentric by 

 covering them partially. With a nasal excentric pupil (i.e., covered on the 

 temporal side) a shifting of the differently coloured images in one direction 

 will be observed ; with a temporal excentric pupil (i.e., nasal side covered) 

 the shifting will be in the other direction. 



Let any one who sees the red letters before the blue "cover his pupils 

 symmetrically on the temporal side, the red letters retreat and soon appear to 

 be behind the blue. On covering the pupils symmetrically on the nasal side, 

 the red letters come forward more and more." 



The bearing of these experiments is fully discussed by Einthoven in the paper 

 already referred to. 



17. Benham's Spectrum Top. 



(a. ) A cardboard circular disc, about 4 inches in diameter, is made with one 

 half black and the other half white. On the white are a number of arcs of 

 concentric circles of different radius. On rotating this disc, coloured lines are 

 seen whose order is reversed when the disc is made to rotate in an opposite 

 direction. The experiment is best performed by artificial light. 



(b.) Modification by Hurst. 



On a circular disc, 4 or 5 inches in diameter, half white and half black, 

 draw in black on the white half and in white on the black half arcs of various 

 lengths and thicknesses, as, for instance, the arcs shown in fig. 283. Mount 

 the disc on a peg and spin it. The arcs appear as circles of various colours, 

 the colour of each depending on its position and length, on the velocity of 

 rotation, and on the kind and intensity of illumination. The two outermost 

 lines on the disc figured when the disc is turned to the left and seen in very 

 bright lamp-light appear purple-grey, becoming, as the rotation becomes 

 slower, brighter and redder, and then in succession bright crimson, scarlet, 

 and orange-vermilion. By very bright direct sunlight the earlier shades are 

 brighter than the later ones, the colour being at first usually a very pure blue. 

 When the disc is turned to the right, the colours are in succession dark green, 

 indigo fringed with pale blue, black, by lamplight, while by bright sunlight 

 the colour is first dull red, then brown, and finally dark blue. They appear, 

 however, very different to different observers. 



The colours of the white lines are almost entirely yellow, orange-pink, puce, 

 and "electric blue." 



If, instead of arcs of circles, a spiral-line is drawn as in fig. 284, the disc 

 exhibits, when spun at a suitable speed, a broad band of colour, consisting of 



