228 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



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

 steady instrument ; it has rack and pinion movement to the coarse 

 adjustment, and sub-stage ; the draw -tube has a mm. scale, and a fine 

 adjustment of the usual Continental type, and all the latest adaptations 

 for sub- stage illumination. The instrument in its simplest form is 

 remarkably low-priced, and the more important apparatus can be 

 added to it 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 fine adjustment is employed, tire 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, save 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 stage 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 fine 

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

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

 features : 



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

 left-hanc 1 side of the limb. 



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

 entirely cut out to the front of the stage ; and 



(3) The body-tube, which was of standard size, viz. 8J inches, 

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

 permitted the use of both long and short tubes. 



This instrument is illustrated in fig. 135. It was also possessed 

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

 Powell and Lealand's dry achromatic combination fitted with a turn- 

 out rotary arm to carry stops. The sub-stage was made by adapting 

 Swift's centring nose-piece, and providing it with a rack and pinion 

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

 graduated stage-plate and sliding bar, a plan devised by Mr. 

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

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

 each should slide into the draw-tube, by which means the diaphragm 

 is in the same place whatever the depth of the eye-piece employed, 

 and it was constructed to do critical work with the highest 

 powers. 



