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focus within the tube, (Fig. 53,) there forming an inverted image. Crossing at this focus, they proceed on to the eye-piece, by the lenses of which they are again refracted, and made to issue in parallel pencils, thus giving a distinct magnified image of the object.

162. The Object-glass. Whenever a beam of light passes through a lens, it is not merely refracted, but it is likewise separated into the different colored rays of the solar spectrum. This separation of the colored rays, or the chromatic aberration, causes the edges of all bodies viewed with such a glass to be fringed with prismatic colors, instead of being sharply defined. It has been found, however, that the chromatic aberration may be nearly removed, by making a compound lens Fig. 54. of flint and crown glass, as represented in Fig. 54, in which A is a

A

concavo

в convex lens of flint glass, and B a
double convex lens of crown glass,-the
convexity of one surface being made to

agree with the concavity of the other lens. The two are pressed together by a screw in the rim of the brass box which contains them, thus forming a single compound lens. When the surfaces are properly curved, this arrangement is nearly achromatic.

The object-glass is placed in a short tube, movable by a pinion attached to the milled head W. (Figs. 51, 52.) By this means it may be moved backwards and forwards, so as to adjust it to dis- B tinct vision.

C

163. The Eye-piece. The eye-piece used in the telescopes employed for surveying purposes consists of two plano-convex lenses, fixed in a short tube, the convex surfaces of the lenses being A

Fig. 53.

D

SEC. V.]

INSTRUMENTS AND FIELD OPERATIONS.

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towards each other. This arrangement is known as "Ramsden's Eye-piece."

164. A telescope with an object-glass and an eye-piece as above described, inverts objects. By the addition of two more lenses the rays may be made to cross each other again, and thus to give a direct image of the object. As these additional lenses absorb a portion of the light passing through them, they diminish the brightness of the image. They may therefore be considered a defect in telescopes intended for the transit or theodolite. A little practice obviates the inconvenience arising from the inversion of the image. The surveyor soon learns to direct his assistant to the right when the image appears to the left of its proper position, and vice versa.

165. The Spider-Lines. The advantage gained by the telescope in producing distinct vision, would add nothing to the precision of the observations, without some means of directing the attention to the precise point which should be observed in the field of view. The whole field forms a circle, in the centre of which the object should appear at the time its position is to be noted. This centre is determined by stretching across the field precisely in the focus of the eye-piece a couple of spider-lines or fine wires, at right angles to each other. The former are generally employed. When they are properly adjusted in the focus they can be distinctly seen, and the point to be observed can be brought exactly to coincide with their intersection. The magnifying power of the eye-piece enables this to be done with the greatest precision. When it has been effected, a line through the centre of the eye-piece and the centre of the object-glass will pass directly through the object. This line is called the line of collimation of the telescope.

The spider-lines are attached by gum to the rim of a circular ring of brass placed in the tube of the telescope at the point indicated by the screw-heads a, a, (Figs. 51, 52,) some of which are invisible in the figure. These screws

serve to hold the ring in position, as represented in Fig 55, and to adjustit to its proper position. The eyepiece is made to slip in and out of the tube of the telescope, so that the focus may be brought to coincide exactly with the intersection of cross-wires. The perfect adjustment of the focus may be determined by moving the

Fig. 55.

eye sideways. If this motion causes the wires to change their position on the object, the adjustment is not perfect: it must be made so before taking the observation.

166. Spider-lines are generally used for making the "cross-wires," though platinum wires drawn out very fine are preferable. The wire is drawn to the requisite degree of fineness by stretching a platinum wire in the axis of a cylindrical mould and casting silver around it. The compound wire thus formed is then drawn out as fine as possible and the silver removed by nitric acid. By this means Dr. Wollaston succeeded in obtaining wire not more than one thirty thousandth (0) of an inch in diameter. As such wire is very difficult to procure, the spider-threads are generally substituted. The operation of placing them in their proper position is thus performed. A piece of stout wire is bent into the form of the letter U, the distance between the legs being greater than the external diameter of the ring. A cobweb is selected having a spider hanging at the end. It is gradually wound round the wire, his weight keeping it stretched: a number of strands are thus obtained extending from leg to leg of the wire: these are fixed by a little gum.

To fix them in their position, the wire is placed so that one of the lines is over notches previously made in the ring. The thread is then fixed in the position with gum or some other tenacious substance. The wire being removed, the line is left stretched across the opening in the proper position.

167. The Supports. Attached to one of the horizontal plates, usually the index-plate of the instrument, are two supports, K, K, (Figs. 51, 52,) bearing the horizontal axis L. These supports should be made of precisely the same height, so that when the plate is level the axis may be horizontal. In some instruments there is an arrangement for raising or depressing one end of the axis so as to perfect the adjustment. In most cases, however, the adjustment is made perfect by the maker, and, if found not to be so, it must be remedied by removing the support which is too high and filing some off from the bottom. This should always be done by the manufacturer.

In the transit the telescope is attached immediately to the axis; but in the theodolite the axis bears a bar M at right angles to it. This bar carries at its ends two supports, which from their shape are called Y's, in the crotch of which the telescope rests, being confined there by an arch of metal passing over the top. This arch is movable by a joint at one side, and is fastened by a pin at the other. By removing the pin and lifting the arch the telescope is released and may be taken from the support. It rotates freely on its axis when confined by the arch. The telescope, being attached thus to the horizontal axis, admits of being elevated or depressed in a vertical plane so that it may be directed to any object.

168. The Vertical Limb. In the theodolite, the vertical limb E consists of a semicircle of brass graduated on its face and attached to the bar M. This limb moves with the telescope upon the horizontal axis, and thus by means of the index F, serves to determine the angle of elevation of the object. In the transit with a vertical circle, the circle is attached to the end of the axis, as seen at E, the index then being attached to the support K. In some instruments, instead of the axis bearing a circle, an arc of from 60° to 90° is attached to the support, and the index is fixed to the axis by an arm which is either permanently fastened` to it or is capable of being clamped in any position.

169. The Levels. Attached to the horizontal plate are two levels A and A set at right angles to each other, so as to determine when that plate is horizontal. They consist of glass tubes very slightly curved, the convexity being upward. They are nearly filled with alcohol, leaving a small bubble of air, which by the principles of hydrostatics will always take the highest point. If they are properly adjusted, the plate to which they are attached will, when these bubbles have been brought to the middle of their run, be level, however it may be turned about its vertical axis. To the telescope of the theodolite and also to that of some transits another level G is fixed. This should be so adjusted that when the line of collimation of the telescope is horizontal the bubble may be in the centre of its run.

170. The Levelling Plates. The four screws S, S, S, and S, called levelling screws, are arranged at intervals of 90° between the two plates P, P, which are called levelling plates or parallel plates. They screw into one plate and press on the other. By tightening one screw and loosening the opposite one at the same time, the upper plate, with the instrument above, may be tilted. To allow this motion, the column connecting them terminates in a ball, which works in a socket in the centre of the lower plate. A joint of this kind, called a ball-and-socket joint, allows movement in all directions.

To level the instrument by means of these levelling screws, loosen the clamp, and turn the plates until the telescope is directly over one pair of the screws. Then, taking hold of two opposite screws, move them in contrary directions with an equal and uniform motion, until the bubble in the tube parallel to the line joining these screws is in the middle. Then turn the other screws in like manner until the other bubble comes to the middle of its tube. When they are both brought to this position the plates are level if the instrument is in adjustment. In levelling, care should be taken to move both screws equally. If one is moved faster than the other, the instrument will not be firm, or will be cramped.

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