HELMHOLTZ'S TOP. 



109 



varied modifications. If, for instance, we place on a disc covered with blue 

 and red sectors of equal size, a black disc, of which the sectors are alternately 

 filled in or empty, the disc, as it turns round, will appear blue if the black 

 sectors of the upper disc exactly cover the sectors of the lower disc ; and it 

 appears red, if, on the contrary, the blue sectors are covered with the black ; 

 while in the intermediate positions we obtain different mixtures of red and 

 white, and during the rotation of the disc may vary the colour insensibly 

 by a gentle touch. By dividing the different sectors with broken or curved 

 lines, instead of straight ones, we can produce an arrangement of coloured 

 rings of great variety and beauty. To give the top greater speed, we set it 

 in motion by drawing a string twined round its stem. The simplest method, 

 as shown in fig. 108, consists in the employment of a handle similar to 

 that of the German top. It is a hollow cylinder of wood set into a handle 

 with two circular holes ; and at right angles with these is a groove for the 

 passage of the string. The stem of the top is passed through the holes of 



Fig. 107. M. Helmholtz's top for studying the impression of light on the retina. 



the cylinder, one end of the string is fixed in the small hole in the stem, 

 and is rolled round by turning the top in the hand. The part of the stem 

 on which the string is twisted becomes sufficiently thick for the top to 

 remain suspended to the handle ; then holding it a little above the table, 

 and giving the string a powerful pull, we set the top in motion, and as the 

 string unrolls it falls on the table, where it will continue its rotation for 

 some time. The top represented in fig. 109 is constructed so that the discs 

 may be firmly pressed by the stem, which is necessary in experiments for 

 demonstrating Newton's theory of the mingling of colours. We make use 

 for this purpose of a variety of discs, made of strong paper of different sizes, 

 having an opening in the centre and a slit, as in fig. 1 1 o ; each of the discs 

 is covered uniformly with a single colour ; and if two or more are super- 

 posed, with their slits placed one over the other, we obtain sectors, the size 

 of which we may vary at will, so that we can modify in a continuous 

 manner the proportions of the colours. The most perfect construction is 

 that of BusokTs chromatic top (fig. 1 1 1), which should only be employed for 

 very rapid rotations. The disc, which weighs 5 Ibs., is made of an alloy 

 of zinc and lead, about an inch and a quarter in diameter. The brass axis 



