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to increase power but sacrifice definition ; the apparent aber- 
rations seemed incorrigible, so that this plan was finally 
abandoned in 1864. Although by this means the Pleuro- 
sigma rhomboides was fairly shown to Messrs. Powell and 
Lealand, with their one sixteenth objective (dated 1862), 
they stated this method had been tried long before and re- 
linquished as useless to improve definition. 
Sliding-tubes (made by them for the purpose) were now 
furnished with a ‘‘ universal screw,” in order to admit a 
great variety of single and compound cemented lenses (more 
or less chromatically and spherically corrected) being inserted 
within the draw tube midway between the eyepiece and the 
objective. So, also, whole or parts of objectives were simi- 
larly applied, thus forming a microscope within a microscope 
admitting endless combinations of compensations. 
It now seemed perfectly clear that any attempt to im- 
prove high-power definition must be preceded by the attain- 
ment of a ready and decisive method of ascertaining whether 
the balance of compensations was equal, or, on the other 
hand, over- or undercorrected. The IJmage-test already 
described appeared to. effect this object in the following man- 
ner. The finest glasses, it is well known, are constructed 
upon the principle of balancing compensations, the effect of 
- the posterior combinations when overcorrected compensating 
that of the anterior glasses which are undercorrected. To 
ascertain therefore the indications of the character of a given 
correction (still employing the exquisite images formed by 
“the sixteenth”) wire gauze, forty meshes to the inch, was 
placed in front of a brilliant light; the image of the gauze 
was distinctly visible under the one quarter objective (x 250 
diameters) finely corrected for an uncovered object. 
To ascertain the appearances due to overcorrection, the 
front glasses were removed ; whilst to examine those of under- 
correction the front set alone was employed, the inner glasses 
being removed. 
First result—The image no longer appeared like gauze, 
but displayed (unless the aperture was reduced) extraordinary 
patterns, prismatic, translucent, and, as it were, chequered 
or plaid-like ; all of which were situated entirely above the 
best focal point, and nothing but a confused nebulous field 
below it. 
Second effect.—The employment of the front lenses alone 
now reversed the position of these appearances. 
Readjusting all the glasses, it was then discovered that the 
false images were developed principally below the best focal 
