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of a graduated milled head. These lenses are conveniently 
traversed within the draw tube; and can be brought to bear 
within four inches of the objective, or at a distance of ten 
inches. 
The focal length of the combination forming the aplanatic 
image-searcher may vary from 13 inch to ? of an inch. The 
latter applies more effectively to low objectives when it is 
desirable to obtain extraordinary depth of focal penetration, 
and vision through very thick glass'—as with a half inch 
giving 700 diameters with a C eyepiece. A Wray half-inch 
objective” was found to bear an E eyepiece and searcher. 
It should now be stated-that the searcher may be employed 
with very different intentions. Thus— 
When it is desirable to view an object through a very thick 
refracting medium, the searcher is brought as close as possible 
to the objective, which action lengthens the focus of the 
objective ; and the same thing is necessary when the observer 
wishes to throw the etdola of an upper structure above and 
away from the true image of the lower but contiguous stratum 
—as when the lower beads of the Podura are required, or 
when it is required to give additional negative aberration to 
an objective too positively corrected in which the front glasses 
are already forced into a dangerous proximity. 
On the contrary, when the searcher is traversed the opposite 
way, the objective lenses require to be brought nearer together ; 
the instrument is then more adapted for viewing objects or 
particles lying in the upper plane of a complex structure, 
throwing the ezdola of the lower layer below that layer itself, 
and so leaving the upper stratum less disguised by the false 
images of the lower. 
In intermediate cases, where greater penetration or focal 
perspective is required, with a thin glass cover, the objective 
lenses must be proportionately separated by an increased in- 
terval, the searcher being traversed towards the objective ; 
and in general confused images of both upper and lower strata 
can be obtained by opposite arrangements.’ 
A very interesting refinement upon the corrections for 
chromatic effects may be accomplished by gradually traversing 
either way both searching and objective lenses and closely 
watching the effect. 
1 Nearly one fourth of an inch thick. 
2 With a “ Kelner” two thirds of an inch focal length, a very clear, very 
large, and flat field is presented to the eye, notwithstanding the increased 
power with the searcher. A one and a half inch objective by Ross was used 
generally for a condensing illuminating apparatus more or less stopped off. 
_ 3 Such as separating the objective lenses and traversing the searcher 
further-from them. 
