peculiar Class of Optical Deceptions. 211 



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 

 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 

 produced by the thaumatrope, and the transparency of the 

 revolving 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 back-ground ; 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 

 produce 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 super- 

 posed 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 

 unequal, then total changes of the appearance supervene ; 

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

 comes 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 



