395.—Screw-drivers, Tommy Pins, etc.—A screw-driver and a tommy pin, the last to turn the capstan heads, are placed in the case with the theodolite. Two screw-drivers with proper handles are better, as there are small and large screws. A camel-hair brush to dust the instrument, a piece of wash-leather, a little vaseline, and a small bottle of good watch oil are also very useful. These little refinements are generally kept out to keep down the price of the instrument.
396.—Additional Parts, and Variations in Theodolites.—Illuminated Axis.—4, 5 and 6-inch transits sometimes, and larger instruments always, have the transit axis bored on one side through to the interior of the telescope, as shown on [Fig. 155]. Through the hole a small pencil of light is sent by a lamp l with a plano-convex lens front, to a lens placed in the end of the axis. This, by a slight adjustment of the lamp on its stand, focusses the light upon a small mirror placed within the telescope, which reflects its rays to the diaphragm. The lamp gives a faint light only sufficient to distinguish the webs for night and underground observations. The mirror is about 1/10 inch in diameter, and is generally mounted upon a milled head screw tapped into the trunnion band of the telescope m. The point of the screw is extended as a thin stem into the axis of the telescope, where the mirror is held by it. This arrangement permits the mirror, which is generally made of silver, but is much better of platino-iridium, to be removed for cleaning. The lamp is mounted upon a wooden stand w carried upon a slide n or upon two brass pins direct to the A-frame. The wood is employed in this case to cut off conduction of heat to the near standard from the lamp as much as possible to prevent disturbance of the axis from expansion by heating. The stand may be removed when the lamp is not required and placed in the case. In large theodolites a pair of lamps are used, that the transverse axis may not be heated more on one side than on the other.
397.—The Lamp, which is found so convenient for bringing a star or distant light to read with the webs, becomes difficult to use when the object is very faint, as the light thrown into the telescope by the lamp takes off the effect of blackness of the night sky or that of total darkness. This becomes important in taking observations of small stars, as for instance, the circumpolar stars of the southern hemisphere. In some theodolites, made first for the Sydney Government, the author placed a very small lamp to throw light upon the face of the webs only, making these appear as light lines on a black ground. The reflecting eye-piece, [Fig. 20], will be found to answer very well, and this is a simple, inexpensive contrivance. Any amount of illumination desired may be thrown on the front of the diaphragm, according to the distance at which the light is held from the eye-piece: generally a very faint light only is required.
398.—The author has illuminated the webs front and back by means of a very small (one-quarter candle power) incandescent lamp, which is charged by a portable battery, or a secondary battery where a dynamo is at hand for charging it, and for countries where these cannot be renewed or where the extremes of temperature are too great for their use, he has devised a small hand dynamo for generating the current and a rheostat for controlling the power of the lamp, so that resistance may be employed to reduce the light to the faintest possible glimmer.
The electric lamp is far superior to the old oil lamp and safe to use in gaseous mines; it is far cleaner, does not give out a tithe of the heat, and may be removed from its socket and used in the hand for reading the verniers in a bad light. All the author's modern instruments that are required with illuminated axis are now fitted with electric lamps.
Fig. 161.—Trough needle for transit theodolite.
Larger image
399.—A Trough or Long Compass, used in place of Circular Compass.—A long compass, [Fig. 32], p. 74, is often applied to a theodolite, either upon the top of the telescope, or more generally and conveniently for reading under the limb. In this last case the trough needle is a separate piece, which is only attached to the limb of the theodolite by means of loop slides or bayonet fittings under the limb, when required to take a bearing. The engraving Fig. 161 shows the long compass with bayonet fittings. There are four slots, two of which are shown SS′, which fit in under the heads of round-headed, shouldered screws. The author has somewhat modified this pattern recently by making it slide into grooves.
The trough needle is generally made 5 or 6 inches long, and reads into a short scale of about 10° at each end. The divisions are best placed upon sliding fittings, so that they may be adjusted by four screws from the outside of the box—screws shown AA′. This enables the needle to be adjusted to its own axis, and also to the 0° reading of the horizontal limb of the theodolite. A slide lift to the needle is shown at L. When the same form of compass is used upon large instruments a reader is placed at each end of the needle.