﻿A NEW COMPARATOR. 141 



of sixty diameters and a field of one and two-tenths millimeters. A glass 

 plate graduated into millimeters is adjusted in the microscope tube so that 

 a millimeter here is exactly equal to a tenth of a millimeter in the field, 

 that is, these lines divide the field of the microscope into twelve equal 

 divisions as shown in fig. 1. The glass plate serves as a micrometer 



FIG. 1. 



device ; by its means tenths of a millimeter can directly be read and the 

 variations between the two bars observed, as they are successively brought 

 into the field. For the illumination of the bars a small electric light 

 (not shown in the plate) is mounted just in front of their graduated 

 surfaces during the reading. The zero points of the rods are adjusted 

 by the screws at the top and are then quickly paralleled by passing the 

 microscope downward along the edges of the bars and the lower ends 

 brought into focus by adjusting the clamps. 



This apparatus could be modified to operate satisfactorily in a hori- 

 zontal position. The reason for making it vertical was to have vertical 

 suspension and avoid the irregularities and inaccuracies produced by 

 sagging or slight bending of the bars; also so that only the upper edge 

 of the microscope travel need be perfectly fitted. At a greater expense 

 an absolutely level table could be constructed for a horizontal apparatus, 

 the microscope travel perfectly adjusted, the whole operated in a ther- 

 mostat, and therefore the temperature perfectly controlled. With a 

 horizontal apparatus, the adjustments of the bars could possibly be made 

 more quickly. 



It is thought that the plan of the double freedom of the microscope 

 possesses many advantages over the stationary microscope and traveling 

 track of other comparators, for example the comparator after Abbe. 8 



8 Ztschr. f. Instrument enkunde (1892), 12, 311. 



