296 On a Peculiar Class of Optical Deceptions. [1831. 



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

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

 certain velocity, a perfectly regular tint ; but when visually 

 superposed, there appeared a fixed wheel, having twenty-four 

 spokes, equal in dimensions to the original spokes. Variations 

 of the position 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 

 background 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 

 sufficiently 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 retaining 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 

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

 known. 



But during such impressions, the eye, although to the mind 

 occupied by an object, is still open, for a large proportion of 

 time, to receive impressions from other sources ; for the original 



