179 
applied, according to the nature of the research in hand. 
The indications of the one adjustment should be employed 
to verify those of the other. Correlative movements by the 
aid of the searcher may introduce aplanatic images, whilst a 
violation of their correlation will exhibit deformity. 
In every case either an extra thickness of glass cover 
or a deeper immersion of a given object in the film of 
Canada balsam (or other fluid used for mounting it) was 
found to require for a precise definition additional ad- 
justment; the searcher should be made in this case to 
traverse towards the object to attain the new correction 
requisite. The same remark is applicable to immersion 
lenses. Further slight improvement can be effected in the 
precision of definition by separating more or less the com- 
ponent glasses of the Huyghenian eyepiece (the power of 
which is preferred as low as 3-inch focal length for the 
zy “immersion’’) or by substituting for it a single achro- 
matic combination slightly overcorrected for spherical aber- 
ration of two inches focal length, or less according to the 
power required.! 
An additional cap containing a supplementary achromatic 
lens is sometimes advantageously fixed upon the lenses of 
the searcher, when (for instance) a power of 700 diameters 
is desired to be developed by a half-inch objective (for test 
Podura beading). 
In conclusion, the experiments detailed in this paper, 
selected from a great number made within the last few years, 
it is hoped will induce more able observers to repeat them in 
amore general form; but, so far as they are detailed, they 
appear satisfactorily to demonstrate the detection of residuary 
aberration of considerable amount in the very finest micro- 
scopes, and enable one to measure it, and to suggest means 
of diminishing the errors of the glasses whilst greatly in- 
creasing the power. Whether a similar method can be 
applied also to telescopes has been some time under the 
author’s consideration, with results which he hopes on a 
future occasion to have the honour of communicating to the 
Society. 
1 Dr. Pigott adds in a note that a Wray one fifth, made expressly, 
admitted of as great amplification as an ordinary one twelfth. In fact, 
these researches appear to point decisively to greater advantages to be 
expected from raising the quality of the lower objectives rather than 
deepening focal length. Observers are more numerous every year who 
prefer the one eighth to the one twenty-fifth and one fiftieth. 
