210 Mr. Faraday on a 



observed, and which for brevity's sake t refrain from de* 

 scribing. 



"Wheels were then fixed on the machine, consisting of radii 

 or spokes, each having twelve, equal in length and width 

 (Fig. 1). When revolved alone, each wheel gave, with a certain 

 velocity, a perfectly uniform tint ; but when visually superposed, 

 there appeared a fixed wheel, having twenty-four spokes, equal 

 in dimensions to the original spokes. Variations of the posi- 

 tion of the eye, or of the relative velocity of the two wheels, 

 caused alterations similar to those I have referred to with the 

 cog wheels. 



In observing these effects, either the wheels should be black 

 or in shade, whilst the part beyond is illuminated ; or else the 

 wheels should be white and enlightened, whilst the part beyond 

 is in deep shade. The cog wheels present nearly a similar 

 appearance in both cases, though in reality the parts of the 

 spectrum which appear darkest by the one method are lightest 

 by the other. The spoke wheels give a spectrum having white 

 radii, in the first method, and dark radii in the second. Placing 

 the wheels between the eye and the clouds, or a white wall, or 

 a lunar lamp, answers well for the first method ; and for the 

 second, merely reversing the position, and allowing the light to 

 shine on the parts of the wheel towards the eye, whilst the 

 back ground is black, or in obscurity, is all that is required. 

 Strictly, the phenomena should be viewed with one eye only, 

 but it is not often that vision with two eyes disturbs the effects 

 to any extent. ' 



The cause of these appearances, when pointed out, is suffi- 

 ciently obvious, and immediately indicates many other effects 

 of a similar kind, and equally striking, which are dependent 

 upon it. The eye has the power, as is well known, of retain- 

 ing visual impressions for a sensible period of time ; and 

 in this way, recurring actions, made sufficiently near to each 

 other, are perceptibly connected, and made to appear as 

 a continued impression. The luminous circle visible when a 

 lighted coal or taper is whirled round the beautiful appear- 

 ances of the kaleidophone the uniform tint spread by the 

 revolution of one of the spoke or cog wheels already described 

 -r-are a few of the many effects of this kind which are well 

 known. 



