172 ILLUMINATION IN MICRO-METALLOGRAPHY: 



due to the ocular and camera length. Like any eye-piece micrometer, 

 its actual value needs to be calibrated against a stage micrometer, 

 but it is available with any eye-piece. 



There remains the difficulty of "glare." The worst source of 

 this trouble is reflection from the surfaces of the objective lenses. The 

 upper convex surfaces are the strongest reflectors. Fortunately the 

 condition that the illuminating beam should fall as if it proceeded from 

 the image plane, means that it is made up of divergent rays which a 

 convex reflector cannot bring to a focus, but reflects with an increased 

 divergence. Consequently with the light focussed correctly only a 

 small spot of glare light appears at the apex of the upper objective 

 lens. The bulk of the reflected light is scattered to the tube sides. 

 Obviously the objective mount and body tube should be well blacked 

 inside. Reflections from the inside of lamp bulbs and other stray light 

 may give trouble. It is best to keep all these away by a screen, which 

 may be the mount of the auxiliary lens or aperture. Another source 

 of glare is reflection from the front lens of the objective. It is only 

 troublesome with dry objectives of short working distance. With 

 those of 2/3 inch and over it is not serious, but it is hardly possible 

 to get negatives with good contrast with dry objectives of 1/4 or 1/6 

 inch. Perhaps this is one reason for the fact that very little metal 

 work is done at magnifications between 150 diameters, the upper limit 

 of a 2/3 inch or 16 mm. objective, and 700 diameters corresponding to 

 a 1/12 inch or 2 mm. oil immersion. One can, of course, get inter- 

 mediate magnifications by using low ocular and camera length with 

 an oil immersion, but the field covered is too small for the general 

 view required. The Zeiss 3| mm. or 1/7 inch oil immersion fills 

 the gap very well. I have tried to get English firms to make a similar 

 lens, and one maker listed a 1/6 inch oil immersion for metallurgical 

 purposes before the war, but has ceased to make it. I would suggest 

 that a 6 mm. or 8 mm. oil immersion should be mad© for the work. 

 The working distance need not be too great for the oil contact, no 

 cover glass has to be allowed for, conditions favourable to giving the 

 objective a relatively large N.A. without introducing specially great 

 manufacturing difficulties. The Zeiss lens mentioned was quite cheap, 

 and of excellent performance. A one inch or 2/3 inch of about .30 

 N.A., a 1/3 or 1/4 inch oil immersion of about .70 N.A., and a 1/12 

 inch of 1.3 to 1.4 N.A. would furnish a metallographer with a bat- 

 tery serving all the purposes. 



For low power work there is room to put the vertical illuminator 

 below the objective, and it can be arranged to give either vertical oi* 

 oblique light. Even here it will be found advantageous to use an 

 image of the radiant formed by an auxiliary lens. 



To sum up the above, it may be said that the items in which 

 improvement is desirable are the following: — 



1. A transparent vertical illuminator which shall get nearer the 

 theoretical perfection of reflecting 50 per cent, and transmitting 50 

 per cent, of the light incident on it at 45°, without colouring the 

 transmitted light. Optically worked glass lightly platinised seems 

 the most promising. 



2. A light source of uniform and steady high brilliancy presenting 

 an area of half-an-inch square or a little more, to which a condenser 

 of 2 inch working distance can be focussed without damage from 



