. The microscope and its revelations. ecting inwards. When thescrew is withdrawn, the rings can be slipped aver the milled heads of the coarse adjust -FIG. 307.—Powell and Lea- , , . ,, , lands protecting ring for mentjand 7 screwing the small screw homecoarse adjustment. the ring cannot be withdrawn ; but as thcv are loose upon the milled heads, the latter cannot be brought into action; the rings simply revolve upon theheads without bringing them into play. Other forms of the same appliance have been made by this firm ;and Messrs. Beck have made these rings with slight modificationsmore recen
. The microscope and its revelations. ecting inwards. When thescrew is withdrawn, the rings can be slipped aver the milled heads of the coarse adjust -FIG. 307.—Powell and Lea- , , . ,, , lands protecting ring for mentjand 7 screwing the small screw homecoarse adjustment. the ring cannot be withdrawn ; but as thcv are loose upon the milled heads, the latter cannot be brought into action; the rings simply revolve upon theheads without bringing them into play. Other forms of the same appliance have been made by this firm ;and Messrs. Beck have made these rings with slight modificationsmore recently. They are the most efficient means of counteractingthe danger incident on public exhibition of delicate objects underhigh powers. The foregoing constitute, it is believed, all the most importantpieces of apparatus which can be considered in the light of accessoriesto the microscope. Those which have been contrived to affordfacilities for the preparation and mounting of objects will be describedin a future chapter (Chapter VI.).. 353 CHAPTER V OBJECTIVES, EYE-PIECES, THE APERTOMETEB IT is manifest that everything in the form and construction as wellAS in the nature of the optical and mechanical accessories of themicroscope exists for, and to make more efficient, the special workof the objective, or image-forming lens combination, which constitutesthe basis of the optical properties of this instrument. The development of the modern objective, as we have alreadyseen, has been very gradual; but there are definite epochs of verymarked and important improvement. Our aim in the study ofobjectives is practical, not antiquarian, and we may avoid elaborateresearches on the subject of non-achroniatic lenses and reflect//;/specula, which have been sufficiently indicated in the third chapterof this volume. We may also pass over the earlier attempts atachromatism ; the true history of the modern objective begins from the,time that its achromatism had been finally worked out. The first
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