210 
BRIDGMAN, ON INDEX TO SCALE. 
and a quarter square (upon which the paper scale is after- 
wards to be fixed), is now made to rest over the right hand 
end of the slide_, but without touching it_, lying upon the 
stage-bar at the bottom_, and the end bar or stop, at the side. 
Instead of being a fixture in this position, three steadying 
pins, soldered into the under side of the plate, a, fit into 
corresponding holes, 1, 2, and 3, fig. 2, in the stop and stage 
bar, and keep it perfectly steady and tight, and yet, by means 
of the handle, c, fig. 3, allow it to be taken off or replaced 
in an instant. A channel or groove leading to the left-hand 
hole, 3, fig. 2, of the stage bar, serves as a guide for directing 
the pins to their proper places, and if a medium thickness of 
slide be used, the plate may be fitted so close upon it as not to 
be in the way of the object-glass. Upon the brass plate, a, 
fig. 3, fasten the paper scale, b, either with paste, gum, diamond 
cement, or shell-lac. The hole, b, in the papered slide, being 
in the field, and the slide close to the stop and the stage bar, 
put the centre of the scale carefully under the point of the 
index, and see that the horizontal lines are parallel with the 
lower edge of the slide, by passing the scale backwards and 
forwards under the index ; or, in this way its place may be 
accurately ascertained and marked beforehand. 
As covers are seldom required larger than three quarters 
of an inch in diameter, the scale, b, need be only nine tenths 
of an inch square ; eight tenths being used for the divisions, 
and one tenth at the bottom and right hand side being 
occupied by the numbers. The divisions being fiftieths of 
an inch, there will thus be only 40 each way, and the centre 
wiU read 20, 20, the first number referring to the top of the 
scale, and the right-hand number to the right-hand side of 
the scale. 
As a finder, in its application to an instrument devoid of 
stage movements, an additional loose stage plate, similar to 
fig. 2, will be required to carry the scale and the stop, but 
used merely as a marker, the usual sliding stage-bar will 
suffice. 
A piece of common looking-glass, about an inch square, 
G, fig. 1, attached to the body of the instrument on the left 
side, by a split ring, may be made to throw light from the 
lamp upon the label or scale, and render the marks legible 
without interfering with the object-glass. 
Although where this instrument has been already applied, 
the same form and position have been retained, it by no means 
follows that other arrangements may not be found equally 
if not more convenient ; as, for instance, the adjusting 
screws, d, n, fig. 1, may be made to work with milled nuts. 
