Larger image
230.—When two levels are at command, one a Y-level, or even a dumpy in perfect adjustment, the one may be used as a collimator to the other by setting them up at a distance within their focal range on a firm basement floor. A candle or a lamp will give sufficient light to illuminate the webs of the instrument, which is used as a collimator, being certain, of course, that this instrument is first placed in level adjustment and set at solar focus, and that the instrument used as a collimator has a good object-glass.
Fig. 69.
Larger image
Fig. 70.—Stanley's model 14-inch dumpy level.
Fig. 70A.—Tripod.
Larger image
231.—Improved Dumpy Level.—The writer has made some improvements in the dumpy level, which have so far met with very general approbation from the profession, Fig. 70. These improvements are directed to ensure much greater sensitiveness in the longer bubble, therefore greater accuracy in the work performed by it; more solidity of construction without increase of weight; and permanence of reading index, with some additional matters. In these improvements the mounting of the longer level tube, instead of being placed in a stiff joint at one end, or between rigid clamping nuts at both ends, has a barrel-fitting at one end which is ground into a parallel hole. This plan admits of circular self-adjustment to the bubble tube, which the clamping of the nuts can never twist or strain during vertical displacement; and the joint can be made perfectly sound with certainty, which saves the risk of accident to the bubble from expansion by heat and some other conditions. A more recent form of cross level, Fig. 69, shown in perspective near the ray-shade in the engraving Fig. 70 has been designed by the author, in which the level casing is bored entirely out of the solid. It is supported upon the side of one telescope strap by three stout pins, the centre one fitting its hole, and the two outer ones are loosely held by cross screws to permit a small amount of adjustment, which is all that is necessary. By this construction the level fixings are made in five pieces only, including screws, instead of thirteen as usual, at the same time making the level more portable and solid for hard wear. The telescope straps are fitted at their stumps solidly down upon the limb, as shown [Fig. 60, p. 112]. Adjusting screws are placed under this as in the dumpy level described, but the pressure screw is not employed except in case of accident far away from an optician, when it is found to be there ready for use. The limb is framed out into two edge bars: this gives greater vertical sectional strength and resistance to torsion without increase of weight in the instrument. Where a compass is used, this is included in the frame of the limb, as shown in the engraving. The compass is read with a prism, this being much more convenient and exact than looking down upon the divided circle, the instrument being necessarily placed for use at nearly the height of the eye. The compass ring is made of aluminium.