BRIDGMAN, ON INDEX TO SCALE. 
207 
either end)^ will be represented^ in exactly a corresponding 
position the counterpart of any spot in the one must of 
necessity fall within the area of the other. In our finder^ this 
distance being first determined upon^ a slide prepared for the 
purpose^ is transferred to the instrument, which then becomes 
the medium for measuring a similar distance upon any other 
slide. The arm_, a, fig. 1, carrying the index, is made a fixture to 
the instrument, and is brought to the right side of the object- 
glass as the most convenient position. The index itself, b, 
fig. 1, is a piece of steel wire, about one tenth of an inch in 
diameter, and an inch and a half long, sliding up and down 
in a brass tube, c, at right angles to the end of the arm, a, 
and vertical to the stage plate. It is also adjustable by the 
two screws, d, d, which unite them. This wire is filed to a 
point, notched at the bottom with two grooves crossing each 
other, and may be raised or lowered by the cross pin, e, 
working in guide slots at the upper end of the tube, which is 
also cut and sprung at the lower end to prevent lateral 
motion. The point is thus kept always in the same relative 
position, and may be easily adjusted by the following pro- 
cess : 
Take a plain glass slide, and paste or gum a strip of paper 
upon it, about two inches and a half in length, as in fig. 2, a. 
Fig. 1. 
