28 Modern Microscopy 



case, and it is made as small as possible. Owing to the 

 extended use of Continental oculars and objectives, English 

 makers have in recent years adopted the Continental 

 diameter of body to a considerable extent. For photographic 

 purposes it is held by some workers that a fairly large body 

 should be used, but for ordinary visual work we do not 

 consider there is any real advantage in the large over the 

 small tube. The growing system of providing a large outer 

 body with an inner draw-tube to carry the small students' 

 eyepieces, is a commendable step, and meets all needs. It 

 may here be said that instruments of Continental type have 

 their bodies constructed very much shorter than the so-called 

 English models, the rule being to employ objectives adjusted 

 for a tube 160 millimetres long with the former, while 

 English opticians usually adjust theirs to one of 250 milli- 

 metres. The microscopist who enters enthusiastically into 

 his work invariably has objectives of both Continental and 

 English make, and he therefore requires the convenience 

 of being able to use both perfectly. One or two English 

 opticians make microscopes with a body of 160 millimetres, 

 and a draw-tube sliding inside it, by means of which a 

 length of 250 millimetres can be obtained ; but a greater 

 range is often found convenient, and Watson and- Sons in 

 their Van Heurck microscope (shown in the frontispiece), 

 and Mr. Baker in his Nelson model microscope, supply two 

 draw-tubes giving a range of body from 140 millimetres 

 (SJ inches) to 310 millimetres (12 inches). One of these 

 draw-tubes works by a rack and pinion : the object of this 

 being to afford facility for adjusting the objective for thick- 

 ness of cover-glass, as described on page 68. This form of 

 body is coming more and more into use, and will be found 

 a very great convenience to the all-round worker. No 

 precise advice can be given without knowing the work 

 intended to be done, but, generally speaking, the short 

 body with the two draw-tubes is much to be preferred to 

 any other. 



The draw-tube usually has a scale of divisions engraved 



