176 THE MICROSCOPE IN ENGINEERING WORKS 



of three colours of the spectrum at one point instead of two in the 

 achromats, a great improvement is effected. This, with the correction 

 for spherical aberration in two colours, gives an image of greater sharp- 

 ness for either white or monochromatic light. 



For low power work, however, and for certain subjects on higher 

 power work, a flatter field and better results can often be obtained by 

 achromatic objectives, as the larger aperture of the apochromats tends 

 to give a slight curvature of the image, which even the compensating 

 or projection ocular cannot entirely correct. 



At the same time it is felt that even the best made German objec- 

 tives do not give enough magnification for micro-photography, as the 

 very fine structure of some alloy steels are at present most difficult to 

 resolve, and much that is now impossible to see might be brought out 

 under higher magnifications. Microphotographs Nos. IX. and X. 

 show " etch figures " in crystals of pure nickel. These serve to 

 determine the crystalline system to which nickel belongs. The crystal 

 in Photograph No. IX. shows a twinning plane, and the consequent 

 difference of orientation as shown by the " etching pits." It is im- 

 possible to say what internal structures might be brought out in heat- 

 treated alloy steels by higher magnifications, as the crystal grains are 

 so much smaller than those of the nickel shown in Photographs IX. 

 and X. 



For metallographic work the following provisions on thei micro- 

 scope seem, to the writer, to be advisable for good work. The micro- 

 scope should be usable in the horizontal position. 



The stage should have a mechanical movement in two directions, at 

 right angles to each other. The stage should also have a racking 

 motion for focussing, as it is usually more suitable to rough focus by 

 this means in preference to that on the tube, as it does away with the 

 necessity of altering the position of the optical bench. The fine adjust- 

 msdnt is usually on the tube of the microscope, and this is the most 

 convenient place. 



Ordinary Huygenian eye-pieces are most suitable for achromatic 

 objectives, while for the apochromatic objectives special compensating 

 eye-pieces are necessary. For photography a projection eye-piece 

 gives the best results. 



The disc illuminator gives most satisfactory results and, with a 

 diaphragm between it and the source of light, good definition can be 

 obtained. 



The objectives as mentioned previously should be used according 

 to the subject — the achromats give every satisfaction for the general 

 run of metallurgical work and, even in photography, are often prefer- 

 able to the apochromats, by giving a flatter field. However, when 

 photographic records of very fine detail are desired there is no doubt 

 of the superiority of the apochromats for the purpose. 



It seems to be desirable to be able to obtain much higher magnifica- 

 tions than are at present obtainable by the present objectives, but, in 

 all probability, improvements in the methods of polishing the specimens 

 will also have to be developed to secure a surface good enough to bear 

 the higher magnification. 



There is undoubtedly a great future before the microscope in its 

 application to engineering work, in relation to designs, steel and its 

 heat-treatment. 



