134 THE METALLURGICAL MICROSCOPE 



If the fine adjustment of the microscope is of the ordinary type 

 in which the head has only a very small longitudinal niotion, the 

 tube, slots, and spindle mentioned above also need only be very 

 short. On the other hand, in the type of microscope designed by 

 the author, in which the fine adjustment may be moved through 

 considerable distance by the coarse focussing of the stage, the tube, 

 slots, and spindle must have a length of several inches. This focussing 

 device, which is somewhat difficult to describe in words, is very 

 simple and efficient in action. It is illustrated in the photograph, 

 Fig. 3. 



Finally, reference may be made to another matter which some- 

 times gives difficulty in metallurgical microscopy. This is the 

 mounting of specimens with their surfaces accurately at right angles 

 to the optic axis of the microscope. Mechanical levelling deyices of 

 various kinds have proved more or less successful, but they all have 

 the serious disadvantage that the carefully prepared surface of the 

 specimen must be placed in contact with some part of the apparatus, 

 and when this is done there is considerable risk of damaging the 

 surface. The author, therefore, has devised an optical levelling 

 appliance in which the surface of the specimen is utilised as a 

 reflector. The specimen is approximately mounted on a glass slip 

 by means of plasticine, wax or other soft substance. It is then 

 placed under the instrument, and its position is adjusted with the 

 fingers until the reflection is seen opposite a cross-wire. When this 

 position has been obtained, the specimen is accurately level, and 

 the manipulation is so easy that it rarely occupies more than five 

 seconds. A more detailed description of this device has been given 

 in the author's paper on '' Some Appliances for Metallographic 

 Research."* 



* Journ. Institute of Metals, igis, I. 



