VERTICAL ILLUMINATORS, METALLURGICAL MICROSCOPES 91 



fixed position, assuring no disarrangement of the proper align- 

 ment of the illuminator with reference to the radiant. 



Most of the large metallographs are developments of the type 

 first suggested by Le Chatelier. Two instruments have been 

 selected for illustration as embodying the largest number of 

 good features to the exclusion of those which are distinctly bad. 

 These have been described at length in preference to other 

 valuable instruments since the author has had the opportunity 

 of working with them and thoroughly testing them. 



The Bausch & Lomb Metallographic Microscope. 1 — The 

 most satisfactory of the large inverted (Le Chatelier) type 

 metallurgical microscopes at present purchasable in the American 

 market is that shown in Figs. 40 and 41, pages 92, 93. 



This instrument consists of an optical bench B 200 cm. long 

 on short legs; the intention being that it will lie upon a shelf 

 suspended from the ceiling with " damping " springs to pre- 

 vent vibration. The bed B carries sliding stands upon which 

 are mounted the various parts of the apparatus as seen in the 

 illustrations. The radiant placed at the far end of the bench 

 consists of a direct current hand feed arc lamp R with hori- 

 zontal carbons, these carbons are very small the + carbon being 

 5 mm. in diameter and the — carbon 4 mm. The manufacturers 

 claim that the substitution of these tiny carbons for those which 

 have been commonly employed adds greatly to the efficiency 

 of the instrument. Attached to the lamp housing is a condens- 

 ing lens C which may be focused by the handle h. The char- 

 acter of the light from R may be modified by light filters inserted 

 in the support S. This support may also serve to hold ground 

 glass or a cell to hold water for cooling, or other liquids. Adjust- 

 ing screws s, s, serve to properly align the rays from the arc. 

 In front of the condenser is an iris diaphragm which serves to 

 cut down the aperture and aid in obtaining a flat field. Between 

 the arc lamp and the microscope is placed a screen E provided 

 with a second condensing lens c also provided with iris dia- 

 phragm. This auxiliary condenser projects a brilliant image 



1 Made by the Bausch & Lomb Optical Co., Rochester, N. Y. Model of 1920 

 designated as Microscope ICF. 



