600 Dll. G. W. EOYSTON-PIGOTT ON A SEARCHER FOR APLANATIC IMAGES. 
field of view, by a proper selection of lenses whilst the optical compensations were 
being adjusted. The form finally adopted is simply this: — 
A pair of slightly overcorrected achromatic lenses, admitting of further correction by 
a separating adjustment, are mounted midway between a low eyepiece and the objective, 
so as to admit of a traverse of 2 or o inches by means of a graduated milled head. 
These lenses are conveniently traversed within the draw tube ; and can be brought to 
bear within 4 inches of the objective, or at a distance of 10 inches. 
The focal length of the combination forming the aplanatic image-searcher may vary 
from 1-|- inch to f 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 7 00 diameters with a C eyepiece. I possess a 
Wray half-inch objective f which bears 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 
eidola of an upper structure above and away from the true image of theTower but con- 
tiguous stratum — as when the lower beads of the Podura are required, or when it is re- 
quired 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 eidola 
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 
interval, the searcher being traversed towards the objective; and in general confused 
images of both upper and lower strata can be obtained by opposite arrangements J. 
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. 
The most brilliant definition is generally obtained when the searcher (a little more 
overcorrected) is used as close to the objective as possible. 
The overcorrection of the searcher is increased by separating its component lenses 
according to the divisions upon the sliding tubes of the searcher. 
* Nearly one-fourth of an inch thick. 
f With a “ Reiner ” two-thirds of an inch focal length, a very clear, very large, and flat field is presented 
to the eye, notwithstanding the increased power w r ith the searcher. A one-and-a-half-inch objective by Ross 
was used generally for a condensing illuminating apparatus more or less stopped off. 
t Such as separating the objective lenses and traversing the searcher further from them. 
