J. W. Bailey on a Universal Indicator for Microscopes. 63 
serve that if the Indicator be placed upon the stage and accu- 
rately centred, with its guide line C, D, parallel to the front edge 
of the stage, and aslide be then placed upon the Indicator so that 
its horizontal guide line shall coincide with C, D, and the right 
hand vertical guide line stand at 70, (¢.e. in the position which 
would be recorded as 32’,) or its left hand guide line at £2”; then 
a motion of the stage itself bearing with it the Indicator and slide, 
or an equal motion of the slide upon the Indicator and fixed 
her, the distance between the guide lines upon the slide agrees 
accurately with that between 40 and 70 of the Indicator, the slide 
when placed upon. the moveable stage at either $3’ or $4’ will 
heed no.displacement for the whole series of numbers; but if this 
distance does not agree, the slide must be put with its left hand 
away with. , 
here are some objections, but not insuperable ories, to the 
moveable stage Indicator as above described. In the first place 
the stage as usually made has its motion too limited to corres- 
pond to the whole range of the Indicator, and secondly the guide 
lines ruled upon the stage for one object-glass may not answer for 
other powers on account of slight inaccuracies of mounting. 
_ The stages can doubtless be constructed to give as wide a range 
for motion as required, which will do away with the first men- 
tioned objection. The second may be removed by placing an 
Indicator upon the upper plate of the stage when the latter stands 
at $4 and adjusting it so that when well centred for the power 
employed the line C F shall be parallel to the front edge of the 
stage. The slide being then placed upon the Indicator with its 
ide li ; 
accuracy in the position of the guide lines upon the slides is done 
Suide lines at 32/ or 5. the remaining motions may be made with 
