NEW BOOKS, WITH SHORT NOTICES. 131 



in tlio upper plane of a complex structure, throwing the eidola of the 

 lower layer below that layer itself, and so leaving the uj)per stratum 

 less disguised by the false images of the lower," 



" In intermediate cases, where greater penetration or focal persj)ec- 

 tive 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.* 



" A very interesting refinement upon the corrections for chromatic 

 effects may be accomplished by gradually traversing either way both 

 searching and objective lenses and closely watching the effect. 



" The most brilliant definition is generally obtained when the 

 searcher (a little more over-corrected) is used as close to the objective 

 as possible. 



" The over-correction of the searcher is increased by separating its 

 component lenses according to the divisions upon the sliding tubes of 

 the searcher. • 



" The change in the general aberration is shown by the divided 

 index of the milled head actiiating the movement of the searcher. 



" The power obtained is in general from two and a half to foiu* 

 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.f 



" 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 conjiigate portion or integral part of the objective itself, in 

 which the minute traversing adjustment of the objective lenses finds 

 its counterjiart in- the more extended and therefore more delicate 

 adjusting traverse of the searcher itself. So that, in short, dm-ing 

 minute microscopical research each adjustment should be intelligently 

 applied, according to the nature of the research in hand. The indica- 

 tions of the one adjustment should be employed to verify those of the 

 other. Correlative movements by the aid of the searcher may intro- 

 duce aplanatic images, whilst a violation of their correlation will 

 exhibit deformity." 



It would appear from this paper, that the very finest definition 

 and working power is to be sought rather in the use of comparatively 

 low objectives, such as the J th, ith, and ith. In a note the author 

 states (p. 602) : — " That aWray ith made expressly, admitted of as great 

 amplification as an ordinary -^^^h. In fact, these researches appear to 

 point decisively to greater advantages to be expected from raising the 



* "Such as separating the objective lenses and traversing the searcher 

 farther from them. 



t " With a ' Kelner ' two-thirds of an inch focal length, a very clear, very large 

 and flat field is presented to the eye, notwithstanding tlie increased power with 

 the searcher. A l|-inch objective by Ross was used generally for a condensing 

 illuminating apparatus more or less stopped otf. " The usual power of the ith 

 with a C eye-piece is 800 ; a power of 4000 is given by an eye-piece of one-fifth of 

 an inch focal length." 



