201.—It is sometimes inconvenient to adjust out of doors: this may be performed very well indoors. By daylight a small cross may be made with ink on a sheet of white writing-paper for the sighting object, which should be placed at as great a distance as convenient, say 20 or 30 feet. By night a pin-hole may be made through a piece of paper and a candle or a lamp be placed behind it.
202.—Adjustment of Vertical Axis.—For this the eye-piece is first brought to focus on the webs. The telescope is then placed directly over one pair of parallel plate screws opposite each other, and the instrument is levelled. The Y's are then opened out; and the telescope is directed so that the intersection of the webs cuts or covers any distinct small mark upon a distant object, or preferably upon the centre reading of a foot line upon a levelling staff. There is no objection to adjusting slightly to this by the parallel plate screws, as this adjustment is independent of the level of the instrument. The telescope is then taken out of its Y's and is turned end for end and replaced. The telescope is now turned half a revolution on its vertical axis, and the webs are again brought to read on the staff, if one is used. If they now fall upon the same spot or foot line, the vertical axis is perfectly perpendicular to the axis of the telescope in this direction. If the webs do not fall upon the first reading or point, the amount of difference of reading is recorded and this space is bisected; so that now, if the telescope be adjusted by the milled head M, at its bearings upon the limb upon which it is supported, for the webs to cut the bisection, the axis will be perfectly perpendicular in the direction of its bearing socket. The same process must now be repeated with the telescope placed at right angles to its first position, that is by bringing it over the other pair of parallel plate screws which were not used at first. There is at all times a certain amount of disturbance of the instrument due to handling it; it is therefore necessary to repeat the whole of the above process until the instrument reverses in any direction, but this final adjustment is better deferred until the adjustment of the level tube, to be next described, has been made.
203.—Adjustment of the Level Tube.—The telescope is placed as before over an opposite pair of parallel plate screws, and these are adjusted until the bubble is in the centre of its run. The telescope is then turned half a revolution, so that it is placed over the same pair of screws in the reverse direction, and the displacement from the bubble from the centre is now noted. The capstan-headed bubble screws at the end of the level B are then adjusted to one-fourth of the difference observed, and the parallel plate screws are adjusted for the other fourth, so that by these two adjustments the difference of the run in the two positions is bisected. The same process is repeated over the second opposite pair of parallel plate screws. If this be very carefully done with a correctly divided bubble, the Y's of the telescope may be opened out and the telescope be reversed end for end in its Y's, and the bubble remain true. But it is quite as well to go over all the adjustments a second time, as before recommended.
204.—If the level is to be adjusted by night, this can be done very correctly by a fine cross drawn on paper placed on a wall, with a candle or gas burner shining brightly on it at twenty feet or so distance from the instrument. For this adjustment by night the instrument must be well constructed, as the tubes require drawing out to their full extent for focussing near objects. If the tubes are not quite straight, the object-glass suffers considerable displacement in the drawing out, or technically droops, which is a very common fault in badly-made instruments.
205.—Where webs are used for the reading, they are liable to become baggy or dirty, art. 101, and very frequently to break; nothing can, therefore, be more useful than to be able to re-web a stop in the evening, with command of the easy and certain means of readjustment described, when far from the optician's aid.
206.—As the Y-level is so perfect in its arrangement for adjustments, and so nearly meets the optician's ideal, it will be well to inquire what are the objections made to its use by the majority of British surveyors. The first and most important is that it possesses so many loose parts, to which the practical man honestly objects. The author was, many years ago, when Y-levels were more popular, trying to persuade a cautious practical surveyor who appeared to be very anxious for the certainty of his work, and who was going abroad, to take a Y-level instead of a dumpy one he was selecting, when he had his arguments stopped by the following question:—"Suppose you were surveying in a tropical country, thousands of miles and an ocean voyage from civilisation, where your native porter objected to carry much weight, and your instrument case had to be left at a back station—when your umbrella was all the burden you felt you could support. In this case, suppose your porter, whom you had lost sight of for a short time, arrived with your level, minus the telescope—lost by becoming loose, perhaps from having been played with while he was resting—how would you praise the Y-level?" This gentleman assured me that he did not, and that this was a true account of his experience with the last Y-level he possessed. Other objections, besides loose parts, are that Y's and collars do not remain as perfect as when they leave the optician—that they are liable to wear by friction of constant movement in being carried about upon the points in contact between them, and thereby form facets; that the collars become corroded by exposure, and that they have open spaces that collect sand from flying dust which fixes itself into the collars and Y's, so that this arrangement loses the perfection the optician claims for it. Further, that the cross bubble, which is uniformly placed on the dumpy level, effects a great saving of time over swinging the telescope backwards and forwards with every movement of the adjusting screws. Another feature is that in the dumpy level, to be described, the vertical and horizontal webs of the diaphragm cannot be disturbed from their position by rotation of the telescope after the level is once set up; and this verticality indicates conveniently at once whether the staff is held vertically, which is otherwise a great difficulty with the ordinary form of Y-level reading.
207.—Improved Y-Level.—The above-described defects the author has tried to remedy by a modification of the Y arrangement, by forming the Y's with much broader bearings, and instead of the old loose pins screw fastenings are fitted, which firmly lock the telescope in position with the webs vertical. This, so far, obviates the danger from loose parts, as by this arrangement the telescope also becomes practically firmly fixed. In adjustment the collars are opened out, and in closing press a stud into the telescope by which it takes a given position. This enables a cross bubble, shown on Fig. 57, to be also placed on the telescope for approximate adjustment, which saves the frequent disturbance of the telescope by making cross adjustments. The diaphragm of this Y-level is exactly the same as that of the dumpy, to be described art. 210. From the limb downwards the author uses the same construction as he now employs on his improved dumpy level. This will be described with that instrument further on, art 231, seq. Also the setting-up adjustment with it, which is different from that already described where parallel plates are employed.
Fig. 57.—Improved Y-level.
Larger image