Fig. 1357.
Fig. 1358.
Fig. 1359.
Another method of testing an end with a line measure is as follows: The bar to be measured is shaped as in [Fig. 1357], the end measurement being taken at a, and the projection b at each end serving to preserve the end surfaces a from damage. The standard bar is then set upon the machine and its horizontal and vertical lines adjusted in position as before described. In connection with this adjustment, however, the bar to be tested is set as in [Fig. 1358]; c being a block of metal (having marked centrally upon it horizontal and vertical lines), placed between the bar and the fixed stop u, its vertical line being in line with the vertical line on the standard. This adjustment being made, the block c is removed and placed at the other end of the bar, as shown in [Fig. 1359], when, if the end measure on the bar corresponds with the line measure on the standard, the vertical line at the other end of the standard will correspond with the vertical line on block c.
To prove that the vertical line is exactly equidistant from each end of the block c, all that is necessary is to place it between the bar and the fixed stop u, [Fig. 1350], adjust the microscope to it and then turn it end for end, and if its vertical line is still in line with the spider web of the microscope it is proof that it is central on the block, while if it is not central the necessary correction may be made. It is obvious that it is no matter what the length of c may be so long as its vertical line is central in its length.