30 Modern Microscopy. 



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 

 (5J 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 41. 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 six-inch 

 body with the two draw-tubes is far preferable to any 

 other. 



Some cheaper students' microscopes, instead of being 

 provided with a rack and pinion for the coarse adjustment 

 of the object-glass, are made with the body to slide in a 

 fixed tube. This is a very rough-and-ready arrangement, 

 and accuracy of centring cannot be maintained as with a 

 rack and pinion. These instruments are, however, largely 

 used in hospitals and medical schools, and possessing one 

 element of advantage namely, cheapness other considera- 

 tions are often made subsidiary. Instruments of this kind 

 are generally cast aside or disposed of in favour of an in- 

 strument having rack and pinion after a very short time, 

 and anyone purchasing a microscope with a view of adding 

 apparatus to it would be well advised in having one with a 

 little less apparatus, but with a rack and pinion instead of 

 a sliding body. 



TAILPIECE AND MIRRORS. 



The mirror should be hung on a tailpiece which can be 

 swung aside. This is usually the case, and is a very great 



