EECHEATIYE SCIENCE. 



01 



of the " Microscopical Journal" of January 

 lastj the following particulars are given of 

 the simplest use of the instrument :— 



Let A (Fig. 1) be a disc having the three 

 primary colours, yellow, red, and blue, evenly 

 distributed in sectors of 120^ on its surface, 

 and let this disc be fixed to the colour-top and 

 rotated. Let b be a second disc, blackened 

 on its upper surface, and having a central 

 aperture somewhat larger than the spiadle 

 of the top, as well as a pattern of six or 

 eight rays cut completely through and out of 



on B is effaced. But if the motion of b is 

 retarded, and at the same time broken up 

 into a seriesofrapidandregidar jerks, ormore 

 properly isochronous vibrations, whUe the eye 

 is held vertically over the spindle, each pat- 

 tern is retained for an instant before the 

 eye, yet suflBciently long to form a distinct 

 image; and owing to the rapidity of the 

 vibrations, a whole circle of images is thus 

 portrayed on the retina of the eye before the 

 image from the first vibration is effaced. In 

 like manner, the colours of the disc A, which 



FlO. 2; 



its substance, so that when held vertically 

 over the disc a it will admit of the colours 

 being seen through such pattern when viewed 

 from above. If now the wheel is set in mo- 

 tion, and the disc b allowed to drop down 

 upon the spindle of the colour-top, it will be 

 held in contact with the spindle by centrifu- 

 gal force, and will revolve in a plane parallel 

 with the wheel, in the same direction and 

 with the same velocity. In this case the 

 colours on a appear mixed, while the pattern 



are perceived only as they are transmitted 

 through the open designs in the disc B, 

 appear in their primitive purity or unmixed 

 state, the colour in one sector being reflected 

 through a given ray of the pattern before the 

 arrival of the colour from that sector by 

 which it is immediately succeeded. Hence, 

 both pattern and colours appear multiplied, 

 thus producing the combinations. 



From the construction of the instrument, 

 about six revolutions of the disc A occur to 



