262 THE HISTOEY AJND DEVELOPMENT OF THE MICEOSCOPE 



parts of a microscope will commend itself to all workers of large and 

 broad experience : 



1. A coarse adjustment by rack and pinion. 



2. A sub-stage. 



3. A fine adjustment. 



4. Mechanical movements to sub-stage, i.e. focussing and centring. 



5. Mechanical stage. 



6. Rack- work to draw- tube. 



7. Finder to stage, 



8. Plain rotary stage. 



9. Graduation and rack- work to rotary stage. 



10. Fine adjustment to sub-stage. 



1 1 . Rotary sub-stage. 



12. Centring to rotary stage. 



This table gives in order the relative values of the several parts ; 

 thus a microscope with a rack-and-pinion coarse adjustment and a 

 sub-stage is to be preferred before a microscope with a rack-and- 

 pinion coarse adjustment, a, fine adjustment, but no sub-stage. Or a 

 microscope with a coarse adjustment by rack and pinion, a sub-stage, 

 and a fine adjustment, is to be preferred before one with the same 

 coarse adjustment and a mechanical stage movement, but no sub- 

 stage or fine adjustment ; and so on. The last item is of least 

 importance, and the importance of all the others is in the order of 

 their numeration. 



Another matter of some significance to the tyro is the relative 

 value, from the point of view of time consumed, and therefore of 

 prime cost, in producing the several kinds of microscopes. The 

 No. 1 stands of half a dozen makers may be near the same cost, but 

 may nevertheless have involved the consumption of very different 

 quantities of the highest class of skilled labour in their production. 



Manifestly the first thing to be looked at in a microscope making 

 any pretensions to quality is the character of the ivorkmanship ; and 

 this should carry with it the question how much machine, and how 

 much hand work and fitting there is in it. Arcs graduated on 

 silver, for example, are very attractive, and with many are most 

 impressive ; but they are simply machine work, and quite inex- 

 pensive. 



In the two great types of models, the bar movement and the 

 Jackson limb, the bar movement involves more than double the 

 actual hand-fitting ; while a fine adjustment with a movable nose- 

 piece takes twice the fitting of one in which the w r hole body is moved 

 by the fine -adjustment screw. In the same way a mechanical stage 

 which is made of machine-planed plates, sliding in a machine-ploughed 

 groove, is much less costly in time and quality of labour than a hand- 

 made sprung stage. So a sub-stage having a movable ring pressed 

 by two screws against a spring has very far less work, and work of 

 a lower class, than one with a true rectangular centring movement. 



It will follow, then, that a Jackson-limbed microscope with no 

 movable nose-piece, with a machine-made mechanical stage and a 

 movable ring for sub-stage, will not have involved more, perhaps, 

 than a third of the skilled work which must be expended on a well- 



