Improved Metallurgical Microscope. By Walter Rosenhain. 153 



this ring the mountings of the various reflectors can be slipped ; 

 the reflectors are thus held independently of the objective, so that 

 the fitting of adapters to various objective mounts is obviated, 

 while the height and position of the reflectors can be varied at will. 

 For general oblique illumination a silver parabolic Lieberkuhn is 

 frequently used, and this is mounted to fit upon the outside of the 

 ring at the lower end of the slide just described. With this 

 Lieberkuhn in position, a fitting for other reflectors may be placed 

 inside the ring carried by the slide, and to this fitting any of the 

 reflectors already described for use above the objective, may be 

 readily fitted. 



Although the parabolic Lieberkuhn is much used for general 

 oblique illumination, and certainly gives some very pleasing 

 effects, the author regards it as a somewhat undesirable form of 

 illuminating appliance, because its indications are very difficult to 

 interpret ; the light is focused upon the object from a great 

 number of directions, and all surfaces inclined in such a way as to 

 throw any of this light into the objective accordingly appear more 

 or less bright. For this reason the author prefers to employ 

 oblique light falling upon the specimen — if not as a strictly 

 parallel beam, yet at least from one general direction. Eotation 

 under such lighting frequently enables the observer to form correct 

 judgments as to the relative heights and inclinations of various 

 portions of the field. Sometimes the relative orientation of 

 different crystals, or other surface markings, may be shown in a 

 very striking manner by the simultaneous use of three beams of 

 oblique light incident upon the specimen from three different 

 directions, and distinguished from one another by their colour. 

 For this purpose beams of light from three sources may be focused 

 upon the specimen and coloured glasses interposed ; in the present 

 instrument this is facilitated by means of three slides for holding 

 the coloured glasses or films, these slides again fitting the three 

 sets of dovetail grooves in the body- tube. 



Eye-piece Focusing Attachment. — When a visual eye-piece is 

 used for purposes of photo-micrography, it is usual to focus the 

 objective on the field visually, the objective being arranged to work 

 at its proper tube-length under those conditions ; the alteration of 

 focus which is required to yield a real image on a screen is then 

 obtained by altering the distance between the objective and the 

 object, thus tending to throw the primary image formed by the 

 objective to a point further from the eye-piece end of the tube. 

 In doing this the objective is caused to work at a tube-length 

 which is considerably shorter than that for which it is best 

 corrected, so that its optical performance is impaired to some 

 extent. If the eye-piece in question is of such a construction that 

 the real image from the objective is actually formed in the tube 

 outside the eye-piece, then the proper method of changing the focus 



