THE PHILOSOPHICAL TOP. 197 



is by causing a disk with different sec- 

 tions of it, or, rather, different sectors, 

 colored differently, to revolve rapidly, 

 by means of a top, for example, so as 

 to mingle and blend the colors in their 

 impression upon the retina of the eye. 

 Of course, in both these last experiments, in order to se- 

 cure complete success, it is necessary that the colors to be 

 combined should be of the same hues and in the same pro- 

 portions as those developed in the spectrum by the decom- 

 position of the pure white light of the solar beam. 



A top of suitable form, as affording a ready means of 

 producing a rapid rotation, answers very well for making 

 experiments in the blending of colors. Indeed, with a lit- 

 tle ingenuity, a top may be contrived so that different disks 

 may be fitted to it, and thus a variety of experiments may 

 be made. The method which Newton adopted, however, 

 was somewhat more systematic than this. He constructed 

 a little machine to which his disks could be fitted, and thus 

 made to revolve very rapidly by means of a multiplying 

 wheel that is, a large wheel turning a small one by. a 

 band. 



The figure on the left, on the next page, represents the 

 disk divided into sectors by lines drawn from the centre 

 to the circumference, the several divisions being painted in 

 the colors which it is desired to blend. "When this disk is 

 put upon the little axle made to carry it, in the machine, 

 and set in rapid revolution, if the colors are of the right 

 hue and properly proportioned, they all disappear, and the 

 whole surface becomes apparently white, as shown in the 

 central figure of the following engraving. 



In process of time, as the solar spectrum was more close- 

 ly examined, and as the instruments for producing it were 

 made more perfect, and the arrangements for performing 



