130 THE METALLURGICAL MICROSCOPE: 



provide. The requirements themselves are mainly those common 

 to all microscopic work of the most exacting kind. In regard to 

 the provision of the most critical definition, the highest possible 

 resolving power and the largest and flattest field, together with the 

 greatest possible approach to freedom from colour and the elimina- 

 tion of differences of actinic and visual focus, hitherto tHe best 

 appo-chromatic lenses have provided the nearest approach to a 

 fulfilment of these requirements. Metallurgical progress, however, 

 undoubtedly tends increasingly to the production of materials having 

 an extremely minute micro-structure, and the differentiation of 

 these and the reading of their life history from their structure, makes 

 increasing demands upon the resolving power of our lenses. The 

 provision of a resolving power which should allow the employment 

 of a much higher useful magnification becomes, therefore, of very 

 considerable practical importance. Whether or not such an achieve- 

 ment is within the range of possibility is a matter for the optician 

 rather than the metallurgist. The difficulties of the problem must, 

 however, be very fully recognised ; one of the most important, no 

 doubt, resides in the difficulty of finding an immersion liquid, of 

 very much higher refractive index than the cedar-wood oil commonly 

 employed. The use of monobromonaphthalen© immersion objectives 

 has been tried, but they do not appear to have achieved any wide- 

 spread use. An effort has also been made to meet this requirement 

 by the use of light of much shorter wave-length. The author has 

 spent a considerable amount of time in endeavouring to use the Zeiss 

 ultra-violet microscope for metallurgical purposes, and has succeeded 

 in obtaining a few micrographs by this means. He has, however, 

 abandoned his efforts, because the expenditure of time required was 

 much too great, while the results themselves were not particularly 

 satisfactory. One of the main difficulties in his experience arose 

 from the internal scattering of the ultra-violet light and the occur- 

 rence of fluorescence within the microscope tube. Even should it 

 be possible to overcome these difficulties, a process which is entirely 

 photographic, and in which the systematic visual examination of 

 relatively large area of specimens is impossible, does not promise a 

 very large range of utility. 



Reverting to the requirements for objectives of the ordinary type 

 intended for metallurgical use, there is one point which requires 

 special emphasis and attention. Clear images, whether visual or 

 photographic, can only be obtained if serious reflections of light from 

 the back surface of the objectives can be avoided. As has been 

 indicated above, this is partly a question of careful adjustment of 

 the light and of the illuminator. With the best of facilities in that 

 direction, however, the author's experience has shown very clearly 

 that different lenses of the same focal length differ very widely in 

 respect of this matter of internal reflections. This appears to be 

 a question of the shape of the back lens of the objective, and especially 

 of the outer surface. Where this is plane it appears to be possible 

 to catch the whole of the reflected light on the mirror or prism of 

 the illuminator, but where the back surface is convex this becomes 

 impossible, and a milky image is very apt to result. 



In regard to eye-piece requirements for metallurgical work, these 

 do not appear to differ from those of other microscopical purposes; 

 there is, however, from the user's point of view, a distinct objection 



