DR. WHITE S MICROMETER. 
40 
somewhat impairs the definition of the object. The definition 
is still farther impaired by the glass on which the lines are 
ruled, which covers the entire field of view. The lines them- 
selves exhibit a sensible breadth, and the exact j uxtaposition of 
the lines and object are not so easily secured, yet for low pow- 
ers this micrometer answers a good purpose. 
AYith Mr. Jackson’s micrometer and the negative eye-piece, 
more accurate results are obtained, and although the fact that 
the micrometer glass covers the whole field, impairs its useful- 
ness with high powers, yet its measurements are as accurate as 
are required for ordinary observations. 
68. I>r. White’s Micrometer. At the suggestion of Dr. 
White, of this city, we have recently made micrometers of an- 
other very simple form, which have given great satisfaction. 
Fig. 21 represents one of these micrometers. 
Fig. 21. 
A semi-circular piece of glass, A C B, has micrometer lines 
ruled as shown in the figure. This is cemented to the dia- 
phragm of the negative eye-piece, and occupies very nearly 
one-half of the field. The edge of the glass appears as a dark 
line A B, across the field, while A D B, occupying a very little 
more than one-half the field, is entirely unobstructed, as though 
no micrometer were used. This allows the eye-piece contain- 
ing this micrometer to be used for ordinary observations. By 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
4 
