Fig. 1354.
Referring, then, to [Fig. 1352], e is a bed or frame that may be raised or lowered by means of the hand-wheel c, so as to bring the plate s (on which rests the bar whose line measure is to be compared) within range of the microscopes. The upper face of e is provided with raised V slideways, which are more clearly seen in the end view of this part of the machine shown in [Fig. 1354]. Upon these raised Vs are the devices for adjusting the height of the eccentric rollers s3, upon which the bars to be tested are laid, s2 representing one of these bars. To adjust the bars in focus under the microscope, these eccentric rollers are revolved by means of levers s4. At s5 is a device for giving to the table a slight degree of longitudinal movement in the base plate that rests upon the raised Vs; on the upper face of e and at s6 is a mechanism for adjusting the height of that end of the plate s. The base plate may be moved along the raised Vs of e by the hand-wheel d.
To test whether the cylindrical guides are deflected by their own weight or are level, a trough of mercury may be set upon the eccentric rollers s3, [Fig. 1352], and the fine particles of dust on its surface may be brought into focus in the microscope, whose carriage may then be traversed to various positions along the cylindrical guides, and if these dust particles remain in focus it is proof that the guides are level with the mercury surface.
Fig. 1355.
The methods of using the machine are as follows: The standard bar has marked upon its upper face (which is made as true as possible and highly polished) a line b ([Fig. 1355]), which is called the horizontal line, and is necessary in order to set the bar parallel to the cylindrical guides of the machine. The lines a, a, are those defining the measurement as a yard, a foot, or whatever the case may be, and these are called the vertical lines or lines of measurement. Now, suppose we require to test a bar with the standard and the lines on its face are marked to correspond to those on the standard.
The first operation will be to set the standard bar on the eccentric rollers s3 in [Fig. 1352], and it and the microscopes are so adjusted that the spider web lines in the microscope exactly intersect the lines a and b on the standard, when the microscope carriage abuts against the heads h, [Fig. 1352]. The standard bar is then replaced by the bar to be tested, which is adjusted without altering the microscope adjustment or the heads h, and if the spider web lines in the microscope exactly coincide with and intersect the lines a and b, the copy corresponds to the standard. But if they do not coincide, then the amount of error may be found by the micrometer wheel g, [Fig. 1353].