1831.] On a Peculiar Class of Optical Deceptions. 



object looked at is not in the way to act as a screen, and shut 

 out all else from sight; the result is, that two or more objects 

 may seem to exist before the eye at once, being visually super- 

 posed. The schoolboy experiment of seeing both sides of a 

 whirling halfpenny at the same moment, the appearances pro- 

 duced by the thaumatrope, and the transparency of the re- 

 volving cog- or spoke-wheels referred to, in consequence of 

 which other objects are seen through the shaded parts, are 

 all effects of this kind ; two or more distinct impressions, or sets 

 of impressions, being made upon the eye, but appearing to the 

 perception as one. 



So it is in the appearances particularly referred to in this 

 paper : they are the natural result of two or more impressions 

 upon the eye, really, but not sensibly, distinct from each other. 

 If, whilst the eye is stationary, a series of cogs like those repre- 

 sented by the continuous outline (fig. 9) pass rapidly before it, 

 they produce a uniform tint to the eye : and for the purpose of 

 following out the description, let it be supposed the cogs are in 

 shade between the eye and a white background ; the tint is 

 then a hazy, semitransparent grey. If another series of cogs, 

 represented by the dotted outline, and close to the first, so as 

 to give no sensible angular difference in the dimensions of the 

 cogs, pass with equal velocity in the same direction, it will pro- 

 duce its corresponding tint. If the two sets of cogs be visually 

 superposed in part, as in the figure, there will be no alteration 

 in the uniformity of the tint. If the cogs of one set be more 

 or less to the right or left of the other, then the superposed 

 part will approach more or less to the tint of the shaded and 

 uncut part of the cardboard wheel, and be less transparent. 

 But if, instead of the motion being equal, the velocities are un- 

 equal, then total changes of the appearance supervene ; the 

 spectrum (if I may so call it) of the superposed parts becomes 

 alternately light and dark, and the alternations take place more 

 or less rapidly as the velocities of the two sets of cogs differ 

 more or less from each other. 



When the cogs move in opposite directions, the uniform tint 

 which each alone can produce is soon broken up in the super- 

 posed parts into lighter and darker portions, and when the 

 velocities of both are equal, the spectrum is resolved into a 

 certain number of light and dark alternations, which are per- 



