DE. Gr. w. EOYSTON-PIGOTT ON A SEAECHEE EOE APLANATIC IMAGES. 597 
compensations, the effect of the posterior combinations when overcorrected compensating 
that of the anterior glasses which are undercorrected. To ascertain therefore the indi- 
cations 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 uncoverd object. 
To ascertain the appearances due to overcorrection , the front glasses were removed ; 
whilst to examine those of undercorrection 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 image (ascertained by reducing the aperture of the 
microscope*) when the objective was undercorrected, and above it when overcorrected. 
Brilliant images of glittering particles of mercury scattered on black cloth nearly verti- 
cally illuminated, fine gauze 80 meshes to the inch, perforated metal, gold-leaf displaying 
against a brilliant light immeasurably small perforations exposed on a rich malachite 
green ground^, were submitted to be examined in miniature as test-objects. From a 
variety of experiments of this kind the following data were arrived at, to guide preli- 
minary observations: — 
That when any well-defined structure is viewed by the best microscopes, there exist 
eidola% or false images on each side of the best focal point. 
That they are placed principally above or principally below the focal point of centrical 
pencils, according as the glasses are over or undercorrected; and that for a single 
stratum sufficiently thin, these eidola are nearly symmetrically exhibited on both side, 
of the best focal point only when the compensations are perfectly balanced. 
It follows from these results that when a structure consists of two superimposed strata, 
in such close contiguity as to come within the optical limits of the eidola , the false 
images of the lower stratum are liable to be confused and commingled with the true 
image of the upper stratum when the objective is overcorrected, and when it is under- 
corrected the false images of the upper ai'e confused with the true of the lower stratum. 
* The true image is at once seen by reducing the aperture ; for this purpose a system of circular stops was 
applied to the microscope at the part where the objective is attached, admitting an instantaneous change in the 
aperture, and showing remarkable effects produced by change in the escentric aberration. Its mode of attach- 
ment is shown at (3, fig. 1, Plate LII., where it is marked aberrameter. 
t The gold-leaf is mounted on a slide in the ordinary way, and exhibits interesting and instructive phenomena. 
+ E"$w\ov. 
