420 Progress in Science. [July, 



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necessary liability to errors of interpretation. Another series of experiments 

 on the effects of traversing combinations of lenses placed within the body of 

 the microscope suggested the idea of " searching the axis mechanically for 

 aplanatic foci; " the trials ultimately resulted in the "Aplanatic searcher" thus 

 described by Dr. Pigott: — " A pair of slightly overcorrected achromatic lenses, 

 admitting of further correction by a separating adjustment, are mounted mid- 

 way between a low eye-piece and the objective, so as to admit of a traverse of 

 2 or 3 inches by means of a graduated milled head. These lenses are con- 

 veniently traversed within the draw-tube, and can be brought to bear within 

 4 inches of the objective, or at a distance of io inches. The focal length of 

 the combination forming the aplanatic image-searcher may vary from i \ inches 

 to f of an inch. The latter applies more effectively to low objectives, when it 

 is desirable to obtain extraordinary depth of focal penetration and vision 

 through very thick glass,* as with a \ inch giving 700 diameters with a C eye- 

 piece. It should now be stated that the searcher may be employed with very 

 different intentions. Thus — when it is desirable to view an object through a 

 very thick refracting medium, the searcher is brought as close as possible to 

 the objective, which action lengthens the focus of the objective; and the same 

 thing is necessary when the observer wishes to throw the eidola of an upper 

 structure above and away from the true image of the lower but contiguous 

 stratum, as when the lower beads of the Podura are- required, or when it is 

 required to give additional negative aberration to an objective too positively 

 corrected, in which the front glasses are already forced into a dangerous 

 proximity. On the contrary, when the searcher is traversed the opposite way, 

 the objective lenses require to be brought nearer together ; the instrument is 

 then more adapted for viewing objects or particles lying in the upper plane of 

 a complex structure, throwing the eidola of the lower layer below that layer 

 itself, and so leaving the upper stratum less disguised by the false images 

 of the lower. In intermediate cases, where greater penetration or focal per- 

 spective is required, with a thin glass cover, the objective lenses must be 

 proportionately separated by an increased interval, the searcher being traversed 

 towards the objective; and in general confused images of both upper and 

 lower strata can be obtained by opposite arrangements. 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. The use of this instrument will be facilitated 

 by first setting the microscope for ordinary use without the searcher, adjusting 

 an eye-piece, 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 eye-piece. 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. The power obtained is in general from two and a-half to four 

 times greater than that given with the third eye-piece, 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 



* Nearly one-fourth of an inch thick. 



