42 PROGRESS OF MICROSCOPICAL SCIENCE.  [ \yuitix, Microseapial 
and five of Quadrumana. The section of the memoir treating of the 
human kidney is a very elaborate one. 
* Dr. Royston-Pigott’s Aplanatic Searcher.—At the meeting of the 
Royal Society (April 28th), Dr. Royston-Pigott read a long and im- 
portant paper on the above subject, and on the effects of the aplanatic 
searcher in improving high-power definition in the microscope. We 
have not space for a proper abstract of the paper, which, however, will 
appear at some length in the ‘Proceedings.’ The following description 
may, however, be of interest :—The aplanatic searcher described by 
Dr. Pigott is 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 indispen- 
sable to the observation of objects under very high powers. The 
inquiry into the practicability of improving the performance of micro- 
scopic 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 ;1,th to the 
ith, 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 
eye-pieces and most powerful objectives hitherto constructed, that better 
effects could be produced by regulating a more gradual bending or 
contraction of the eccentrical rays emanating from a brilliant micro- 
scopic origin of light. It then appeared that delusive images, which 
the writer has ventured to name eidola,* exist in close proximity to 
the best focal point (where the least circle of confusion finds its locus). 
(I.) That these images, possessing extraordinary characters, exist 
principally above or below the best focal point, according as the objec- 
tive spherical aberration is positive or negative. (II.) That test- 
images may be formed of a high order of delicacy and accurate por- 
traiture 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 conveniently two 
thousand times less than a known original) are formed (under precau- 
tions) with a remarkable freedom from aberration, which appears to 
be reduced in the miniature toa minimum. (IV.) The beauty or in- 
distinctness with which they are displayed (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 corrections suitably manipulated, it is 
practicable to compare known delusions with suspected phenomena. 
* From cléwAoy, a false spectral image. 
