eye, the field, the amplifying, and the object lenses, the whole forming a compound microscope having its focus in the plane of the cross-wire ring BB.

The eye-piece is brought to its proper focus usually by turning its milled end, the spiral movement within carrying the eye-tube out or in as desired; sometimes a pinion, like that which focuses the object-glass, is employed for the same purpose.

101. The Cross-Wires, (Fig. 89), are two fibres of spider-web or very fine platinum wire, cemented into the cuts on the surface of a metal ring, at right angles to each other, so as to divide the open space in the center into quadrants.

102. Optical Axis -The intersection of the wires

FIG. 89.

forms a very minute point, which, when they are adjusted, determines the optical axis of the telescope, and enables the surveyor to fix it upon an object with the greatest precision.

The imaginary line passing through the optical axis of the telescope, is termed the Line of Collimation, and the operation of bringing the intersection of the wires into the optical axis is called Adjusting the Line of Collimation. This will be hereafter described.

103. The Vertical Circle firmly secured to the axis of the telescope is 4 inches diameter, plated with silver, divided to half degrees, and with its vernier enables the surveyor to obtain vertical angles to single minutes.

104. The Level on Telescope consists of a brass tube about 6 inches long, each end of which is held between two capstan-nuts connected with a screw or stem attached to the under side of the telescope tube.

105. The Magnetic Needle is four to five inches long in the different sizes of transits, its brass cup having inserted in it a little socket or center of hardened steel, perfectly polished, and this resting upon the hardened and polished point of the center pin, allows the needle to play freely in a horizontal direction, and thus take its direction in the magnetic meridian. The needle has its north end designated by a scallop or other mark, and on its south end has a coil of fine brass wire, easily moved, so as to bring both ends of the needle to the same level. The needle is lifted from the pin by a concealed spring underneath the upper plate, actuated by a screw shown above, thus raising the button so as to check the vibrations of the needle, or bring it up against the glass when not in use, to avoid the unnecessary wear of the pivot.

106. The Lower Plate, called the Limb, is divided on its upper surface-usually into degrees and half-degreesand figured in two rows, viz., from 0 to 360, and from 0 to 90 each way; sometimes but a single series is used, and then the figures run from 0 to 360 or from 0 to 180 on each side.

107. The Verniers, of which there are two placed opposite each other against the limb, are auxiliary scales used in measuring smaller portions of the limb than are shown by its graduations. Thirty divisions on the vernier correspond precisely with twenty-nine half degrees on the limb. Hence one division on the limb exceeds one division on the vernier by one-thirtieth of one-half of a a degree, that is, by one minute.

Accordingly, the number of any division of the vernier, on the side toward which the vernier is moved, which coincides with a division of the limb is the number of minutes of arc intercepted by the zero of the vernier and the last preceding division of the limb.

Thus, by the device of a vernier we are enabled to measure angles to within one minute, although the limb of the transit is graduated only to half-degrees.

Adjustments.—The principal adjustments of the Tran

sit are

(1) The Levels.

(2) The Line of Collimation.

(3) The Standards.

108. To Adjust the Levels.-Set up the instrument upon its tripod as nearly level as may be, and having unclamped the plates, bring the two levels above and on a line with the two pairs of leveling screws; then with the thumb and first finger of each hand clasp the heads of two opposite, and, turning both thumbs in or out, as may be needed, bring the bubble of the level directly over the screws, exactly to the centre of the opening. Without moving the instrument proceed in the same manner to bring the other bubble to its centre; after doing this, the level first corrected may be thrown a little out; bring it in again; and when both are in place, turn the instrument half-way around; if the bubbles both come to the centre, they would need no correction, but if not, with the adjusting pin turn the small screws at the end of the levels until the bubbles are moved over half the error; then bring the bubbles again into the centre by the leveling screws, and repeat the operation until the bubbles will remain in the center during a complete revolution of the instrument, and the adjustment will be correct.

109. To Adjust the Line of Collimation. To make this adjustment-which is, in other words, to bring the intersection of the wires into the optical axis of the telescope, so that the instrument, when placed in the middle of a straight line, will, by the revolution of the telescope, cut its extremities-proceed as follows:

Set the instrument firmly on the ground and level it carefully; and then having brought the wires into the focus of the eye-piece, adjust the object-glass on some well-defined point, as the edge of a chimney or other object, at a distance of from two hundred to five hundred

feet; determine if the vertical wire is plumb, by clamping the instrument firmly and applying the wire to the vertical edge of a building, or observing if it will move parallel to a point taken a little to one side; should any deviation be manifested, loosen the cross-wire screws, and by the pressure of hand on the head outside the tube, move the ring around until the error is corrected.

The wires being thus made respectively horizontal and vertical, fix their point of intersection on the object selected; clamp the instrument to the spindle, and having revolved the telescope, find or place some good object in the opposite direction, and at about the same distance from the instrument as the first object assumed.

Great care should always be taken in turning the telescope, that the position of the instrument upon the spindle is not in the slightest degree disturbed.

Now, having found or placed an object which the vertical wire bisects, unclamp the instrument, turn it half way around, and direct the telescope to the first object selected; having bisected this with the wires, again clamp the instrument, revolve the telescope, and note if the vertical wire bisects the second object observed.

Should this happen, it will indicate that the wires are in adjustment, and the points bisected are with that of the centre of the instrument, in the same straight line.

If not, however, the space which separate the wires from the second point observed, will be double the deviation of that point from a true straight line, which may be conceived as drawn through the first point and the centre of the instrument, since the error is the result of

D

B

A

FIG. 90.

C

two observations, made with the wires when they are out of the optical axis of the telescope.

For, as in the diagram, let A represent the centre of the instrument, and BC the imaginary straight line, upon the extremities of which the line of collimation is to be adjusted.

B represents the object first selected, and D the point which the wires bisected, when the telescope was made to revolve.

When the instrument is turned half around, and the telescope again directed to B, and once more revolved, the wires will bisect an object, E, situated as far to one side of the true line as the point D is on the other side.

The space, DE, is therefore the sum of two deviations of the wires from a true straight line, and the error is made very apparent.

In order to correct it, use the two capstan head screws on the sides of the telescope, these being the ones which affect the position of the vertical wire.

Remember that the eye-piece inverts the position of the wires, and therefore that in loosening one of the screws and tightening the other on the opposite side, the operator must proceed as if to increase the error observed. Having in this manner moved back the vertical wire until, by estimation, one-quarter of the space, DE, has been passed over, return the instrument to the point B, revolve the telescope, and if the correction has been carefully made, the wires will now bisect a point, C', situated midway between D and E, and in the prolongation of the imaginary line, passing through the point B and the centre of the instrument.

To ascertain if such is the case, turn the instrument half around, fix the telescope upon B, clamp to the spindle, and again revolve the telescope toward C. If the wires again bisect it, it will prove that they are in adjustment, and that the points, B, A, C, all lie in the same straight line.

Should the vertical wire strike to one side of C, the error must be corrected precisely as above described, until it is entirely removed.