LXXIV.] SPECTRUM TOP. 363 



a complete series of all the colours of the spectrum in their normal order, red 

 being on the outer and violet on the inner side of the band when the disc is 

 turned to the left, and in the reverse order when it is turned to the right. 

 The purity of the colours seen depends very greatly on the light used. With 



bright daylight no trace of a spectrum is seen, but a series of colours ranging 

 from purple through brown to green, or other series according to intensity of 

 light and velocity of rotation. Even under the best conditions, namely, 

 bright lamp-light, slow rotation, and the eyes too fatigued to follow the line 

 round or sufficiently practised to remain motionless, the colours are not all 

 brightest at the same moment. The violet has merged into black before the 

 rotation has become slow enough to give the brightest red and orange. 



Beyond the limits of the spectrum-coloured band are two fringes, a purple 

 or violet one beyond the red, and luminous pale blue on the violet side. These 



FIGS. 283 and 284. Modifications of Discs for Benham's Spectrum Top (Hurst). 



fringes, as well as the spectral band, change somewhat in colour as the speed of 

 rotation changes. 



The spiral is most easily drawn with a brush full of black paint, by draw- 

 ing it lightly across a rotating white disc while the disc is spinning. A suit- 

 able portion of the curve is chosen and the other half of the disc is blacked. 

 Dull black paint, such as water-colour " lamp-black," is best. 



A very different colour-band is produced by a similarly shaped spiral curve 

 of white drawn on the black half of the disc. The colours are " electric " blue, 

 pink, yellow, the blue being outermost when the disc is spun to the left- 

 Spirals of various " pitches " may be used, the line itself being not more than 

 one-fifth of the breadth of the space between two successive turns of the spiral. 

 (Communicated by C. Herbert Hurst, Ph.D.) 



The appearances presented when the tops are viewed in monochromatic light 

 are quite as surprising as those described above (see Abney, Nature, vol. 51, 

 p. 292, 1895). 



18. Anaglyph. 



The pictures of one object are printed on one card in different colours, say 

 pale red and blue. The two pictures are slightly displaced relative to each 

 other. On looking at the picture through a blue and a red glass, i.e., a blue 

 glass in front of one eye, and a red one in front of the other, one sees a nearly 

 colourless object, but the whole is stereoscopic. 



