peculiar Class of Optical Deceptions. 209 



to the middle of the wheel, and a needle run through both, and 

 then withdrawn. These wheels could at any time be put upon 

 the axes, and, being held sufficiently firm by the friction of the 

 cork, turned with them. By these arrangements the axes could 

 be changed, or the wheels shifted, or the velocities altered 

 without the least delay. 



The beauty of many of the effects obtained by this apparatus 

 has induced me to describe it more particularly than I otherwise 

 should have done. The appearance which I first had shewn 

 to me by Mr. Maltby was exhibited very perfectly ; two equal 

 cog wheels were mounted (Fig. 6.), so as to have equal oppo- 

 site velocities; when put into motion, which was easily done 

 by the thumb and finger applied to the upper pully of the hori- 

 zontal copper plate, they presented each the appearance of an 

 uniform tint at the part corresponding to the series of cogs or 

 teeth, provided that the eye was so placed as to see the whole 

 of both wheels ; but when a position was taken up, so that the 

 wheels were visually superposed, then in place of an uniform 

 tint, the appearance of teeth or cogs was seen misty, but per- 

 fectly stationary, whatever the degree of velocity given to the 

 wheel. By cutting the cogs or teeth in the wheel nearest to 

 the eye, deeper (Fig. 7.), the eye could be brought into the 

 prolongation of the axes of the wheels, and then the spectral 

 cog wheel appeared perfect (Fig. 8). The number of intervals 

 thus occurring was exactly double the number of teeth in either 

 wheel : thus a wheel with twelve teeth produced twenty -four 

 black, and twenty-four white alternations. When one wheel 

 was made to move a little faster than the other, by shifting the 

 wooden roller on its axis, then the spectrum travelled in the 

 direction of that wheel having the greatest velocity ; and with 

 more rapidity the greater the difference between the velocities 

 of the two wheels. When the wheels were looked at so that 

 they only partly visually superposed each other, the effect 

 took place only in those parts : and it was striking and extraor- 

 dinary to observe, as it were, two uniform tints mingling, and 

 instantly breaking out into the alternations of light and shade 

 which 1 have described. There are many variations in the 

 curvature and other appearances obtained by altering the posi- 

 tion of the eye, which will be immediately understood when 



