132 



THE METALLURGICAL MICROSCOPE 



Beyond the objectives named above, a demand exists, and is 

 becoming increasingly important, as indicated above, for first-class 

 objectives of long focus. The author would welcome such objectives 

 having focal lengths of 30 mm., 50 mm. and 75 mm., suitable mainly 

 for photographic purposes. It would, however, be an advantage if 

 they could be designed to work with a low power eye-piece so that 

 they could also be used for visual work 



The accessories required in metallurgical microscopy are of some 

 importance. A satisfactory illuminant is essential to all good w^ork 

 of this kind. For visual purposes, the requirements are easily met. 



Fig. I. 



since it is only necessary to place opposite to the lateral aperture of 

 the illuminator an uniform source of light having a reasonable area 

 (about 2 centimetres in diameter). Such a source of light may be 

 obtained by placing a suitable burner or electric lamp behind either 

 very finely ground glass or a thin piece of opal shade. If the 

 luminous surface thus produced is placed to one side of the micro- 

 scope in such a position that its distance from the illuminator mirror 

 is equal to the distance from that mirror to the back focus of the 

 objective, the result is an approach to the conditions of " critical 

 illumination," and for visual purposes these are certainly the best 

 conditions obtainable. This arrangement has the further advantage 

 that no lenses, condensers, etc., are required, and that an iris 

 diaphragm placed just outside the illuminator aperture is all that 

 is needed to regulate the illumination. The whole arrangement can 

 be very simply made by mounting the lamp with a short external 

 tube through which the light passes to the illuminator, the rest of 



