136 benham's artificial spectrum top. 



the case when the top is spun in the ordinary way between the 

 thumb and finger of the right hand), the arcs along the portion 

 AC will produce rings more or less blue, or at any rate fringed 

 with blue, and those along CB will produce rings more or less red 

 or fringed with red, and in some parts the background will be of a 

 more or less greenish tint. Thus, with the arrangement shown in 

 the figure, we shall see two blue rings inside, followed by three red 

 rings, and lastly two blue rings outside all. 



If, however, the disc is spun in the opposite direction (as will 

 be the case when it is attached to a turn-table and spun by the 

 left hand) the colours will be reversed, the arcs along AC produc- 

 ing red rings, while the arcs along CB produce blue rings; the 

 blue will, therefore, now be intermediate between the inside and 

 outside red rings. In the top, manufactured by Messrs. N'ewton 

 and Co., there are twelve arcs, arranged in batches of three on one 

 side of the centre and then three on the other, producing alternate 

 groups of three red and three blue concentric rings. As a rule, 

 the best effects are obtained by revolving the disc in a bright light 

 and the colours seen depend on the rate of rotation, being usually 

 brightest when the disc is rotating as slowly as is. consistent with 

 the markings, appearing blurred into continuous rings. 



A series of investigations on these appearances has been made 

 by Prof G. D. Liveing, of Cambridge, who has also constructed a 

 variety of discs with figures in black disposed on a white ground 

 and with white figures on a black ground. All of these, when 

 revolved in a bright light, show remarkable bands of colour of 

 various shades of red, green, and blue. The general result of his 

 observations of these discs is that if a succession of black and 

 white objects are presented to the eye with moderate, but not too 

 great, rapidity, then, when black is followed by white, an impres- 

 sion of a more or less red colour is perceived, while when white is 

 succeeded by black a more or less blue colour is perceived. If 

 the succession of black and white is very rapid, the appearance 

 presented to the eye is of a more or less neutral green or drab. 



For different people the rate of rotation necessary to produce 

 the impression of a particular tint varies somewhat. When excen- 

 tric or spiral bands of black are used, the impression on the eye 

 is that of fringed bands, reddish on one side and greenish on the 



