DR. Or. W. ROYSTON-PIGOTT ON A SEARCHER EOR APLANATIC IMAGES. 601 
It will be seen that an exceedingly small pencil engages the surface of the searcher 
diverging from a point in the image p t q„ which is inverted again at p n q„. As the searcher 
is traversed nearer the eye the pencils become less divergent, and the effect of the searcher 
is diminished. On the contrary, as it approaches the objective, p, q, being formed nearer 
to the latter after refocusing, a more divergent pencil engages a greater aperture of the 
searcher, and this now automatically causes a stronger overcorrection than before. The 
essential action of the searcher is to apply a rapid variable correction by a traversing 
movement (fig. 2, Plate LII.). 
The use of this instrument will be facilitated by first setting the microscope for ordinary 
use without the searcher, adjusting an eyepiece, the focus, and screw-collar to the most 
distinct vision, and then applying the draw tube containing the searcher placed at a point 
nearest to the eyepiece E. As the searcher is traversed towards the objective, the lenses 
of the objective may require separation. 
The change in the general aberration is shown by the divided index of the milled head 
actuating the movement of the searcher (M, fig. 1). 
The power obtained is in general from two and a half to four times greater than that 
given with the third eyepiece C of 1 inch focal length : with a very fine eighth of Messrs. 
Powell and Lealand’s new construction, a clear and satisfactory definition of the beading 
of the Pleurosigma formosum was exhibited to them, by means of the aplanatic searcher, 
at a power estimated at 4000 diameters*. Several inferior objectives have acquired a 
fine definition by the application of the searcher. 
This paper perhaps will hardly be complete if I omit to add, that the instrument will 
be most effectively employed by considering it as a conjugate portion or integral part of 
the objective itself, in which the minute traversing adjustment of the objective lenses 
finds its counterpart in the more extended and therefore more delicate adjusting traverse 
of the searcher itself. So that, in short, during minute microscopical research each 
adjustment should be intelligently 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 correction will exhibit deformity. 
I ought also to state that I have found 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) to require for a precise definition additional adjustment; 
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 component glasses of the Iluyghenian eyepiece (the power of which is preferred 
as low as 3-inch focal length for the -gw “ immersion”) or by substituting for it a single 
* The usual power of the one-eighth with a C eyepiece is 800 ; a power of 4000 is given by an eyepiece of 
one-fifth of an inch focal length. 
