J. W. Bailey on a Universal Indicator for Microscopes. 61 
vat any time. All this is secured by having the Indicator engraved 
upon a steel plate and printed upon cards of uniform quality, and 
by taking the dimensions from the standard United States inch, 
preserved in the office of the Superintendent of Weights and 
Measures in Albany. In order to extend the use of the Indicator 
to all cases which are likely to occur, the graduation was ar- 
ranged with reference to slides three inches long and one inch 
wide, while it will answer equally well for smaller ones. When 
these slides are not covered with paper, and gnide lines can be 
ruled as above directed upon the glass itself, the graduations ne- 
cessary for their use would ouly extend upon the verticals 3 a | 
inch above and below the horizontal line, and upon the horizon- 
tal line only 4 an inch outwards from the points 40 and but 
in order to provide for paper covered, or opaque slides whose up- 
deft. Fora portion of the objects under the cover 
‘ t of axes and numbers may be used at pleasure provided 
at the verticals are chosen as far apart as possible. 
Two verticals on the same side of the centre should never be 
used together, as a small error in observing the numbers would 
have more effect in displacing the object from the centre than if 
two axes at a greater distance had been em ployed. The reason for 
leaving a blank ungraduated space between 50 and 60 on the ho- 
rizontal kine was to allow a fac-simile of the Indicator to be en- 
graved upon the stage of any microscope, the blank space being 
left for the portion of the stage occupied by the aperture. — 
The guide lines upon the glass may be ruled with a fine pointed 
scratching diamond, and be rendered more visible by having 
graphite or black lead rubbed into them. Lines ruled im this man- 
ner will answer for all except very minute objects, but in conse- 
quence of the widening of the lives by the chipping up of the glass 
j e lines often 
employed for the etching as it gives lines wh ich are too smooth 
= difficult to see, a ebb will not retain the black lead if 
. ; 
