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



a grate is moved in the other, then spectral lines, equal to the 

 number of bars in the grate, are produced. If one grate is 

 moved before another, then the lines are proportionably nume- 

 rous ; or if the distances are equal, and the velocity the same, 

 so that many spectral lines may coincide in one, that one is so 

 much the more strongly marked. If the bars used be serpen- 

 tine or curved, the lines produced may be either straight or 

 curved at pleasure, according as the positions and motions are 

 arranged, so as to make the intersecting point travel in a straight, 

 or a curved, or in any other line. 



The cause of the curious appearance produced, when spoke- 

 or cog-wheels revolve before each other, already described, 

 will now be easily understood; the spokes and cogs of the 

 wheels produce precisely the same effect as the bars held in the 

 hand, and the fixedness of the position of the spectrum depends- 

 upon the recurrence of the intersecting or hiding positions, 

 exactly in the same place with equal wheels, provided the 

 opposite motions of each be of equal velocity, and the eye be 

 fixed. 



When wheels were used in the little machine described 

 (fig. 4<), having equal but oblique teeth, and the obliquity in 

 the same direction, the spectrum was also marked obliquely ; 

 but when the obliquity was in opposite directions, the spectrum 

 was marked as with straight teeth. 



When equal wheels were revolved with opposite motions, one 

 rather faster than the other, the spectrum travelled slowly in the 

 direction of the fastest wheel ; when the difference in velocity 

 between the two wheels was made greater, the spectrum 

 travelled faster. These effects are the necessary consequence 

 of the transference of the intersecting points already described, 

 in the direction of the motion of the fastest wheel. 



When one wheel contained more cogs than the other, as, for 

 instance, twenty-four and twenty-two, then with equal motions 

 the spectrum was clear and distinct, but travelled in the direc- 

 tion of the wheel having the greatest number of teeth. When 

 the other wheel was made to move so much faster as to bring 

 an equal number of cogs before the eye, or rather any one part 

 of the eye, in the same time as the other, the spectrum became 

 stationary again. The explanations of these variations will 

 suggest themselves immediately the effects are witnessed. 



