1831.] On a Peculiar Class of Optical Deceptions. 295 



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 other- 

 wise should have done. The appearance which I first had 

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

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

 opposite velocities ; when put into motion, which was easily 

 done by the thumb and finger applied to the upper pulley of 

 the horizontal copper plate, they presented each the appear- 

 ance of a 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 

 a uniform tint, the appearance of teeth or cogs was seen 

 misty but perfectly 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 visually superposed each other in 

 part, the effect took place only in those parts : and it was 

 striking and extraordinary to observe two uniform tints 

 mingling, and instantly breaking out into the alternations of 

 light and shade which I have described. There are many 

 variations in the curvature and other appearances obtained by 

 altering the position of the eye, which will be at once under- 

 stood when observed, and which for brevity's sake I refrain 

 from describing. 



Wheels were then fixed on the machine, consisting of radii 



