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The most brilliant definition is generally obtained when the 
searcher (a little more overcorrected) is used as close to*the 
objective as possible. 
The overcorrection of the searcher is increased by separating 
its component lenses according to the divisions upon the 
sliding tubes of the searcher. 
It will be seen that an exceedingly small pencil engages 
the surface of the searcher diverging from a_ point 
in the image p’ gq’, which is inverted again at p” g”. 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’ g’ 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 IX). 
The use of this instrument will be facilitated by first setting 
the microscope for ordinary use without the searcher, adjust- 
ing 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 satisfac- 
tory 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. 
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 ex- 
tended, and, therefore, more delicate adjusting traverse of 
the searcher itself; so that, in short, during the minute 
microscopical research each adjustment should be intelligently 
1 The usual power of the one eighth with a C eyepiece is 800; a power 
of 4000 is given by an eye piece of one fifth of an inch focal length. 
