260 THE HISTOEY AND DEVELOPMENT OF THE MICROSCOPE 



of the object is important are : (a) When the object is polarised, and 

 then it is a distinct disadvantage not to be able to rotate the object 

 independently of the body which carries the analyser. In short, the 

 stage rotating independently of the body would be preferable because, 

 if it is required to rotate the object on a dark polarised field, the 

 polarising and analysing prisms can be set at the proper angles, and 

 then the object rotated without disturbing the relative positions of 

 the prisms. 



But this cannot be done with the arrangement of the Zeiss 

 model, which rotates body and stage. The firm have, however, 

 more recently introduced a rotating stage based on the English 

 model, and we are glad to give our testimony to its admirable 

 workmanship and perfection of centring. The contention, however, 

 that we think in all friendliness is sustained, is that the charac- 

 teristics of the English model were not superfluous, and that the 

 Continental model has only too slowly followed the requirements 

 discovered and used by the makers of the best English models so long 

 ago. 



((3) For photo -miwographic purposes. In this case, in the Zeiss 

 stand, the head of the fine-adjustment screw is geared to the focussing 

 rod ; so, manifestly, rotation of the body becomes impossible. 



Thus, by adopting rotation in the form chosen, the highest ends 

 for which the microscope stage should revolve cannot be accomplished, 

 and the newer form of stand must be adopted. 



The sub-stage is often quite wanting in the common Continental 

 forms. This was true of the Hartnack stands, with rare excep- 

 tions ; the Nachet instruments were provided with an elementary 

 form. 



As we have seen, until quite recent times, the condenser was 

 regarded on the Continent as a superfluous, if not a foolish, appliance ; 

 but that prejudice has been killed by the light thrown on the whole 

 question by (1) the chromatic (1873), and now (2) the achromatic 

 condenser of Abbe, and finally (3) by the ' centring achromatic 

 condenser,' only just made accessible by this firm. This condenser is 

 not only focussed by the rack-and-pinion movement, but also by 

 means of a special fine adjustment for bringing out its most delicate 

 results. But even a condenser was in use in England in the year 

 1691 (vide fig. 101, p. 133), and the best work in England since the 

 invention of achromatism has never been done without one. 



In the mounting of the Abbe condenser every possible ingenuity 

 has been displayed to make it do its work without a sub-stage ; but 

 a permanent centring and focussing sub-stage, into which this optical 

 arrangement could, amongst others, fit, might be made with half the 

 labour, ingenuity, and cost. But rather than this, we have in the 

 less recent forms the condenser made to slide on the tail-piece, and 

 to be jammed with a screw. 



It has therefore neither centring nor focussing gear ; but, striking 

 as it may appear, a diaphragm, which cannot be used with, and is no 

 part of, the condenser, is supplied in a stand not of the most recent, 

 but of comparatively recent make, with mechanical centring and 

 rack-work focussing movements ! That is to say, the delicate centre 



