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 numerous; 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 serpentine 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 spokeor 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 Iixedness 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 direction 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.
