OF METALLURGICAL MICROSCOPES 137 



racking. This is particularly desirable in students' micro- 

 scopes, as it would prevent a common accident in labora- 

 tories where inexperienced students use the instruments. 



(c) Fine A(/ji/st/KCfif. — This does not call for much remark, as 



there are several good forms in use. The speed is some- 

 times made too great for comfortable focussing of high 

 powers. The side arrangement of small milled heads ia 

 perhaps the most convenient. 



(d) Bodf/ Tube. — This should be of the short Continental form, 



and preferably of wide diameter. The latter condition is 

 essential in instruments to be used for photographic work, 

 and should always be adopted, but it has also great advan- 

 tages for visual observation, and can be introduced without 

 interfering with the general design. 



(e) T/ie Sf((f/e. — A plain stage of fairly large size is suitable for 



most ordinary work. It should be provided with a rack- 

 work focussing movement, but a fine adjustment is un- 

 necessary. A central hole, sufficiently large to allow an 

 objective to pass through it, allows of the examination 

 of heavy specimens resting on the foot, unless the support 

 of the stage be arranged to swing aside entirely, as 

 mentioned above. Levelling stages are a nuisance, and 

 should never be used. The specimen should always be 

 levelled before placing on the stage, either by means of 

 plasticine and one of the usual mounting devices, or by 

 mea,ns of Dr. Rosenhain's auto-collimating instrument. 

 Mechanical movements to the stage are essential for high 

 power work, and rotation is also a very great convenience, 

 but when both are provided the rotation should be con- 

 centric. A rotating plate which is carried by the traversing 

 movements is useless. When a microscope is intended to 

 be used in the horizontal position, it is desirable to provide 

 the mechanical movements with clamping screws, as other- 

 wise a heavy specimen may cause a gradual downward slip 

 during the exposure of a photograph, pulling down the 

 rackwork by its own weight. This has often been noticed 

 when photographing at high magnifications. The rotating 

 circle should have a clamping screw. The Zeiss-Martens 

 stand has a very convenient rotating and traversing stage^ 

 but the range of movement is too limited. 



The examination of fractures, large crystals in ingot 

 sections, and other things requiring very low powers and 

 great distances, is troublesome when an ordinary microscope 

 is used, and it is often preferable to employ a camera 

 with a landscape or copying lens instead of a microscope. 

 The telephoto attachment of the Davidson microscope gives 

 good results in this kind of work, and the arrangement 

 in the recent pattern, by which the object is carried on 

 a separate stand, movable along a base board, is con- 

 venient. On the other hand, the writer does not approve 



