208 
BRIDGMAN, ON INDEX TO SCALE. 
Draw a line parallel to, and half an incli from, the lower 
edge .of the slide. On this line make a minute hole, an inch 
and a half from the right hand end, and, with a pair of 
compasses set correctly to an inch, place one of its points in 
the hole, and, inking the other point to render the spot 
conspicuous, mark the place on the right hand end of the 
slide. There will now be one mark— a minute hole, b, near 
the centre of the slide, and another — an ink spot, c, one 
inch to the right of it. Place this slide upon the stage, 
using the clip to keep it steady in its place, having first 
adjusted the stage plate perfectly square, and the body of 
the instrument, if moveable, well up to the stop. Bring the 
hole, B, into focus, and as near to the centre of the field as 
possible ; then adjust the point of the index, b, fig. 1, accu- 
rately over the centre of the ink spot, and tighten the 
screws, d, d, to fix it in that position. The papered slide 
being removed, and any object mounted on a slide, with a 
plain paper label, r, fig. 1, previously fastened upon it, 
being now brought into the centre of the field, raise the 
style or index, b, fig. 1, and, with a common pen dipped in 
ink, touch its point, press it down gently till it touches the 
paper label for an instant, and a neat, little, round dot will be 
the immediate result. At any future time, by merely placing 
this dot, as 5, fig. 2, again under the point by which it was pro- 
Fig. 2. 
duced, and which may be done in a few seconds, the object 4, 
fig. 2, will of course occupy its former position in the centre 
of the field. 
In arranging a cabinet of Desmidiae, Diatomacese, or any 
