Telescopic Mirrors and Object Lenses. 91 
and be restored to its place in the same relative situation, as 
before. 
All these conditions may be easily accomplished by machi- 
nery, and perhaps with far greater accuracy than by the hand of 
a workman. The convex leaden tool is fixed into the middle 
of a wooden wheel, (abc, Fig. 2,) having a groove in its cireum- 
ference. To the under side of the wheel is annexed a circular 
iron plate, of less diameter, confined horizontally by three knobs, 
upon which the wheel rests. ‘The knobs, (a, b, c,) fixed in posi- 
tion, are screwed to a board; one only bemg moveable for the 
purpose of adjustment. Fig. 3, is a vertical section of the wooden 
wheel, (a, a), with the leaden tool, (6, 6), and the circular iron 
plate, (ce, ¢). 
The mirror and the concave leaden tool are each inserted in 
a circular box, having likewise a groove in the upper part of its 
circumference; the lower part being smooth and flat. Fig. 4, is 
a vertical section of the box (a, a), containing the mirror or concave 
leaden tool (6, 6) One of these boxes, containing the mirror or 
leaden tool, is placed (Fig. 2), on the convex leaden tool, and 
between two thin laths inclined at a small angle, and united into 
a triangle by a cross bar of iron. The smallest angle (d), of the 
triangle (d, e, f;) is made the centre of motion, and the opposite 
iron side is produced both ways to pass through two fixed holes 
(g, h), confining the triangle to move in its own horizontal 
plane. The sides of the triangle press only horizontally on the 
lower parts of the box where it is smooth and flat, (i. e. cylin- 
drical). The motion is communicated to the triangular frame by 
a rod (wv), connected with a crank and fly wheel. The velocity 
of the mirror, moved in this manner, is greatest in passing the 
centre, and increases gradually from the stationary points: such 
a motion is peculiarly adapted for steadiness and equality of stroke ; 
and the momentum of the fly wheel prevents all sticking and 
M 2 
