228 TILE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



class, li has a tripod base on the English model, and is a thoroughly 

 steady Lnstrumenl : it has rack and pinion movement to the coarse 

 adjustment, and snl> stage; the draw-tube has a mm. scale, andafine 

 adjustment oft in- usual Continental type, and all the latest adaptations 

 I'm- sub stage illumination. The instrument in its simplest form is 

 remarkably low priced, and the more important apparatus can be 

 ; ,dded to il as required. It is illustrated in fig. 181. 



Beck's third-class microscope is shown in fig. 182. It has a good 

 tripod foot with a single pillar. The Jackson model is used, but 

 a peculiar line adjustment is employed, the lever being placed below 

 the stage, the position of the screw being immediately behind the 

 pillar which supports the limb, and where it is easy of access. The 

 body is not affected by vibration when it is touched. The lever is 

 of the second order, and it draws down the body limb and coarse 

 adjustment. In fact, stive in its fine adjustment, this form ap- 

 proximates somewhat to the Continental model. The fine-adjust- 

 ment lever is rather short, but it will be found to be much steadier 

 and slower than the direct-acting screw. 



The stage is plain, without mechanical movements ; but it has a 

 movable glass >tage over the principal stage; to this the slip is 

 clipped, and the whole super-stage of glass is moved with ease 

 over a fair area. The aperture in the glass stage is not large 

 enough ; it should be cut right through to the front, which would 

 much increase its usefulness. 



This instrument also has a sub-stage with rack and centring 

 movements. 



Swift and Son's earlier third-class microscope in its most 

 suitable form dates from about the time of the vertical lever line 

 adjustment patented )>\ that firm (q.v.) It was first made from the 

 designs of Mr. Iv .M . Nelson, and it presented three distinctive 

 features : 



(1) The milled head of the fine adjustment was placed on the 

 left-ham' side of t he limb. 



(2) The stage was of a horseshoe form, the aperture being 

 entirely cut out to the front of the stage; and 



('!) The hod\ tube. \\hich was of standard size, viz. 8| inches. 

 was made in two pieces, \\hich not only secured portability, but also 

 permitted the use of hot h long and short, tubes. 



Tliis instrument is illnsl rated in lig. I,",."). It was also possessed 

 of a cheaply made and fairly good centring sub-stage, to carry 

 1'owell and Lealand's dr\ achroinat ic combination fitted with a turn- 

 out rotary arm to curry stops. The suh si age was made by adapting 

 Suift's cent ring nose piece, and providing it, with a rack and pinion 

 focussing arrangement, as illustrated in lig. 183. There was also a 

 ,'jra dualed stage plate and sliding bar. a plan devised by Mr. 

 Wright for a Under. The eye pieces were provided with rings, like 

 IWell and Lealand's, outside the tube to govern the depth which 



i should slide into the dra\\ tube, by which means the diaphragm 



s in the same place whatever the depth of t he eye-piece employed. 



it was constructed to do critical work with the highest 



