Gibbons^ on a New Method of Micrometry. 
33 
A simple formula^ applicable to all cases, is the result. With 
care, and the use of well-made scales, this process is as accu- 
rate as it is simj)le.^^ 
(These were my earlier operations, and I frequently use 
the same modes even now, although they have led me to a 
much better plan.) 
"By thus projecting the image of the micrometer in card, 
or other material, a scale may be made for each combination 
of lenses. The scale so made may then be laid on the stage 
beside the object, and viewed with it at the same time outside 
the instrument with the disengaged eye. The magnified 
image will then bear exact proportion to the enlarged scale 
upon which it is projected, and the measurement, ivhich 'will 
then be actual, m.ay be read off, at once, to a very minute frac ■ 
tion. This process will have special advantage in the case 
of living objects, which may be measured even while in 
motion.^^ 
My mode of making these scales is as follows : I place 
an ordinary stage micrometer, which I keep as a standard, 
on the stage, and, beside it, a long slip of card, glass, or 
metal plate. Viewing these together, as above described, 
I prick off the intersections of the micrometer divisions with 
a line ruled on the card, &c. I then remove the card, ink 
in the divisions, and subdivide them as far as may be ad- 
visable. This is practicable on the enlarged scale to a degree 
that would be confusing, even if possible, on a common mi- 
crometer. I make such a scale for each combination of 
powers, and for such positions of the draw-tube as I find 
convenient {i.e., as give even numbers, &c.) 
My first application of the plan was to one of Oberhauser's 
microscopes. I made nine scales, viz., one for each combi- 
nation of three eyepieces and three objectives. I send here- 
with one of the original scales, by way of sample, as it will 
better illustrate my meaning than more detailed description ; 
of course it will be applicable to any instrument adjusted to 
the same ai umber of diameters. 
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For Oberhauser's, the field of which does not exceed five 
inches diameter, and for other instruments having small 
calibre, the card scales answer very well ; but for my large 
Ross I have them on glass, as the large field, 9i inches 
