CRITICISM OF MECHANICAL PARTS 259 



fifth of the amount lifted by the micrometer screw of the 1889 

 model is lifted by the same screw in the new model. It should be 

 remembered that few makers of microscopes in England, though 

 they may be for class and school purposes, if they use a fine ail just 

 ment at all. use anything less delicate than the Campbell differential 

 screw ; although it seems on the Continent to be believed that the 

 direct-acting micrometer screw of the Continental form is still in 

 vogue. 



It must be plain that a screw of T^th inch to a revolution 

 cannot bear for long the heavy strain of the body of a microscope. 

 The remodelling of Zeiss fine adjustments in 1886 undoubtedly 

 improved their construction and quality of work ; but so fine a steel 

 thread is not meant to carry weight and strain. This applies to all 

 delicate instruments of precision. 



The stage of this instrument, in common with all built on the 

 same model, has three fundamental error* of design : 



i. The stage is so narrow that the edges of the 3x1 slips are. in 

 some Continental stands, allowed to project over the edges. Mo>sr>. 

 Zeiss have profitably departed from this fault by giving to their 

 larger stands a stage in size more like the English type. 



ii. The stages have an aperture so small as to limit their useful- 

 ness in focussing with high powers. 



iii. Instead of a sliding ledge they provide what still more 

 efficiently militates against easy and rapid focussing, viz. spring 

 clips. It is unfortunate that no stage on this model admits of the 

 use of the finger to aid in reaching the focus. This gentle tilting 

 up of the object, as we approach the focal point, would save hundreds 

 of cover-glasses and objective fronts and we have reason to know 

 that not a few are broken with this form of stage ; but we have never 

 seen put forward, and do not know, a single reason in justification 

 of a small aperture in the stage. 



Another important point is the absence of rotation in the 

 ordinary Continental stand. True rotation is a strictly English 

 feature, which has been in use and carefully constructed for many 

 years. And its value is great ; it is an indispensable adjunct to 

 practical work. 



.Messrs. Zeiss, some twenty years since, copied the Oberhauser 

 form of rotation for the stage ; they did this by making the bodv 

 and limb solid with the stage, so that the whole rotates to- 

 gether. 



Practically there is only one point in favour of such a move- 

 ment, and that is, that the object remains exactly in the same 

 position in regard to the field. But against this arrangement there 

 is 



1. The liability of throwing the optic- axis above the stage out of 

 centre with that below the stage, and this though the workmanship 

 be, as it is, of the highest order. 



1. The rotation of a microscope object for ordinary examination 

 is really unimportant, as there can be no top or bottom to it. Even 

 for oblique illumination it is not required, as it is always easier to 

 rotate the illuminating pencil. The only instances in which rotation 



b 2 



