16 
NATURE 
[May 5, 1870 
THE third number of the new Italian Geological Journal, or 
Bollettino, published by the ‘*Comitato Geologico d'Italia,” 
opens with M. Igino Cocchi’s paper on the stratified rocks of 
the isle of Elba. It especially relates to the lower secondary, 
eocene, cretaceous, and post pliocene strata, and is illustrated 
with engravings. Among the bibliographical notices, Professor 
Omboni’s work on the Geology of Italy, with eight maps, is well 
spoken of, ‘The number also comprises translations of extracts 
from foreign memoirs. 
THE American Naturalist, Vol, 1V., No. 2, April 1870.— 
The April number of this Journal contains three exceedingly 
interesting articles, namely, a report on the Sea Otters of the 
north-west coast of America and Aleutian islands, by Captain 
C. M. Scammon ; a paper on Parasitic Insects, from the able 
hand of Dr. A. S. Packard ; and some notes on the Fresh- 
water Fish of New Jersey, by Dr. C. C. Abbott, which con- 
tains many valuable remarks. The last two papers are 
illustrated. —Dr. W. Wood contributes a popular article on 
Falconry.—The usual reviews and short miscellaneous notices 
complete the contents of the number. 
SOCIETIES AND ACADEMIES 
LONDON 
Royal Society.—April 28.—‘‘ On the Organs of Visionin the 
Common Mole.” By Robert James Lee. 
‘On an Aplanatic Searcher, and its Effects in improving 
High Power Definition in the Microscope.” By G, W. 
Royston-Pigott, M.A., M.D. 
The Aplanatic Searcheris intended to improve the penetration, 
amplify magnifying power, intensify definition, and raise the 
objective somewhat further from its dangerous proximity to the 
delicate covering-glass indispensable to the observation of 
objects under very high powers. The inquiry into the practica- 
bility of improving the performance of microscopic _ object- 
glasses of the very finest known quality was suggested by an 
accidental resolution in 1862 of the Podura markings into black 
beads. ‘This led to a search for the cause of defective definition, 
if any existed. A variety of first-class objectives, from 
the +; to the 4, failed to show the beading, although 
most carefully constructed by Messrs. Powell and Lealand. 
Experiments having been instituted on the nature of the errors, 
it was found that the instrument required a better distribution of 
power ; instead of depending upon the deepest eyepieces and 
most powerful objectives hitherto constructed, that better effects 
could be produced by regulating a more gradual bending or re- 
fraction of the excentrical rays emanating from a_ brilliant 
microscopic origin of light. It then appeared that delusive 
images, which the writer has ventured to name e/do/a*, exist in 
close proximity to the best focal point (where the least circle of 
confusion finds its focus). 
I. That these images, possessing extraordinary characters, 
exist principally above or below the best focal point, according as 
the objective spherical aberration is positive or negative. 
II. That test-images may be formed of a high order of 
delicacy and accurate portraiture in miniature, by employing an 
objective of twice the focal depth, or, rather, half the focal 
length, of the observing objective. 
III. That such test-images (which may be obtained con- 
veniently two thousand times less than a known original) are 
formed (under precautions) with a remarkable freedom from 
aberration, which appears to be reduced in the miniature to a 
minimum. 
IV. The beauty or indistinctness with which they are dis- 
played (especially on the immersion system) is a marvellous test 
of the correction of the observing objective, but an indifferent 
one of the image-forming objective used to produce the testing 
miniature. 
These results enable the observer to compare the known with 
the unknown. By observing a variety of brilliant images of 
known objects, as gauze, lace, an ivory thermometer, and 
sparkles of mercury, all formed in the focus of the objective to 
be tested with the microscope properly adjusted so that the 
axes of the two objectives may be coincident, and their cor- 
rections suitably manipulated, it is practicable to compare known 
delusions with suspected phenomena. 
It was then observed (by means of such appliances) that the 
~ From eidwXov, a false spectral image. 
aberration developed by high-power eyepieces and a lengthened” 
tube followed a peculiar law. 
A. A lengthened tube increased aberration faster than it gained 
power (roughly, the aberration varied ‘as v*, while the power 
varied as 7). 
B. As the image was formed by the objective at points nearer 
to it than the standard distance of nine inches, for which the best 
English glasses are corrected, the writer found the aberration 
diminished faster than the power was lost, by shortening the 
body of the instrument. | 
C. The aberration became negatively affected, and required a_ 
positive compensation. | 
D. Frequent consideration of the equations for aplanatism— 
suggested the idea of searching the axis of the instrument for- 
aplanatic foci, and that many such foci would probably be found | 
to exist in proportion to the number of terms in the equations 
(involving curvatures and positions). ‘ 
E. The law was then ascertained that power could be raised, 
and definition intensified, by positively correcting the searching 
lenses in proportion as they approached the objective, at the © 
same time applying a similar correction to the observing objec- | 
tive. 
The chief results hitherto obtained may be thus summarised. — 
The aplanatic searcher increases the power of the microscope 
from two and a half to five times the usual power obtained with 
a third or C eye-piece of one inch focal length. The eighth thus 
acquires the power of a twenty-fifth, the penetration of a one- 
fourth. And at the same time the lowest possible eyepiece 
(3-inch focus) is substituted for the deep eye-piece formed of 
minute lenses, and guarded with a minutely perforated cap. The 
writer lately exhibited to Messrs. Powell and Lealand a brilliant 
definition, under a power of four thousand diameters, with their 
new ‘eighth immersion” lens, by means of the searcher and 
low eyepiece. 
The traverse of the aplanatic searcher introduces remarkable 
chromatic corrections displayed in the unexpected colouring 
developed in microscopic test objects. The singular properties, 
or rather phenomena, shown by eidola, enable the practised 
observer in many cases to distinguish between true and delusive 
appearances, especially when aided by the aberrameter applied 
to the objective to display excentrical aberration by cutting off 
excentrical rays. Eidola are symmetrically placed on each side 
of the best focal point, as ascertained by the aberrameter when 
the compensations have attained a delicate balance of opposite 
corrections. 
If the beading, for instance, of a test object exists in two 
contiguous parallel planes, the eidolon of one set is commingled 
with the true image of the other. But the upper or lower set 
may be separately displayed, either by depressing the false 
eidola of the lower stratum, or elevating the eidola of the upper. 
For when the eidola of two contiguous strata are intermingled, 
correct definition is impossible so long as the aperture of the 
objective remains considerable. 
One other result accrues : when an objective, otherwise 
excellent, cannot be further corrected, the component glasses 
being already closely screwed up together, a further correction 
can be applied by means of the adjustments of the aplanatic 
searcher itself, all of which are essentially conjugate with the 
actions of the objective and the variable positions of its compo- 
nent lenses; so that if dx be the traversing movements of the 
objective lenses, dz that of the searcher, F the focal distance of 
the image from the objective when 6x vanishes, / the focal 
distance of the virtual image formed by the facet lenses of the 
objective, then , 
a ~ (7) 
The appendix refers to plates illustrating the mechanical 
arrangements for the discrimination of eidola and true images, 
and for traversing the lenses of the aplanatic searcher. ‘The 
plates also show the course of the optical pencils, spurious disks 
of residuary aberration and imperfect definition, as well as some 
examples of ‘‘ high-power resolution ” of the Podura and 
Lepisma beading, as well as the amount of amplification 
