Fig. 1352.
[Fig. 1350] is a top view, and [Fig. 1352] a front view, the upper part of the machine being lifted up for clearness of illustration. x, x, are the cylindrical guides, upon which are the carriages i, k, for the microscopes. The construction of these carriages is more fully seen in [Fig. 1351], which represents carriage k. It is provided with a hand-wheel r, operating a pinion in a rack (shown at t in the plan view figure of the machine) and affording means to traverse the carriage along the cylindrical guides. The microscope may be adjusted virtually by the screw m4. The base upon which the microscope stands is adjustable upon a plate n, by means of the two slots and binding screws shown, and the plate n fits in a slideway running across the carriage. u is one of the stops used in making end measurements, the other being fixed upon the frame of the machine at v in the plan view, [Fig. 1350]. The micrometric arrangement for the microscope is shown more clearly in [Fig. 1353]. The screw b holds the box in position, the edge of the circular base on which it sits being graduated, so that the position of m may be easily read. In the frame m is a piece of glass having ruled upon it the crossed lines, or in place of this a frame may be used, having in it crossed spider web lines. These lines are so arranged as to be exactly in focus of the upper glass of the microscope, this adjustment being made by means of the screw s. The lines upon the bar are in the focus of the lower glass; hence, both sets of lines can be seen simultaneously, and by suitable adjustment of the microscope can be brought to coincide.
Fig. 1353.
Beneath the cylindrical guides, and supported by the rack t that runs between and beneath them, are the levers p, in [Fig. 1352], upon which weights may be placed to take up the flexure or sag of the cylindrical guides.
In [Fig. 1352], h, h, are heads that may be fixed to the cylindrical guides at any required point, and contain metallic stops, against which corresponding stops on the microscope carriages may abut, to limit and determine the amount to which these carriages may be moved along the cylindrical guides.
The pressure of contact between the carriage and the fixed stops is found to be sufficiently uniform or constant if the carriage is brought up to the stops (by means of the hand-wheel r, [Fig. 1351]) several times, and a microscope reading taken for each time of contact. But this pressure of contact may be made uniform or constant for all readings by means of an electric current applied to the carriage through the metallic stops on heads h, h, and those on the carriage.
We have now to describe the devices for supporting the work and adjusting it beneath the microscopes.