SOME POINTS CONCERNING SHARPNESS IN 



HIGH MAGNIFICATION MICROGRAPHS. 



By Carl Benedicks and Erik Walldow (University of Stockholm) 



1. Microscope and accessoi'ies used. 



The following will give a short account of some optical studies 

 executed by us, using the new-constructed metallographical micro- 

 scope of C. Reichert of Vienna. 



The most prominent feature of the new microscope, which is 

 constructed according to the Le Chatelier type, is the very con- 

 venient interchangeability of the plain glass illuminator and the 

 prism illuminator. This interchangeability was introduced in con- 

 sequence of a short paper by one of us,* in which evidence was given 

 of the superiority of the former illuminator at high magnifications. 

 Another innovation is the very convenient adjustment, with index 

 and scale, of the position of the prism — which is of the Le Chatelier 

 type, v/ith two reflecting surfaces — so as to enter more or less, 

 according to the focal length of the objective. 



Another detail of the construction is, that the coarse adjustment 

 is operated by a rack and pinion motion of the stage on which the 

 specimen rests (face downwards), whilst the fine focussing is obtained 

 by a micrometer slide motion of the objective and tubes. The 

 advantage is, that a heavy weight on the stage will have no influ- 

 ence on the delicate slide motion. 



For photographic work, a green glass filter was used, giving a 

 rather well-defined wave length of 0.5-0.6 fj-. Orthochromatic 

 plates (Wellington, anti-screen, backed) were used, and an arc lamp 

 of about 350 c.p., the duration of exposure was increased by this 

 filter in the ratio 10:1. The regular exposure (with filter) was 

 20 sees, when the glass slide illuminator, 4 sees, when the prism 

 illuminator, was used. 



The test specimen was a lamellar pearlite of 0.90 per cent, carbon 

 content, polished in bas-relief on parchment. 



2. Arc lamp and incandescent lamp. 



Fig. 1 gives the specimen at a magnification of 1,200 (arc lamp; 

 immersion apochromatic f = 2 mm.. Num. aperture 1,30; projection 

 eye-piece Nr. 2; camera length 65 cm.). 



In Fig, 2 a'' Half- Watt " incandescent lamp of 60 c.p. was 

 used; of course, several advantages are obtained by a less intense 

 source of light. The exposure had to be prolonged 36 times, to 

 12 min. 



The optical quality of Fig. 2 is still good, but the definition is 

 impaired by a general want of sharpness due to vibration during 

 the long exposure. 



It must be pointed out, however, that under quieter conditions 

 photographs were obtained with the incandescent lamp of the very 

 highest sharpness, which in no respect differed from Fig. 1. This 

 proves that the candle poiver of the lamp has no influence on the 

 image quaVity — a point which, though very natural, scarcely has 

 been proved so far as yet. 



'^ C. Benedicks, Metalhirgie, Vol. 6, p. 320, iqoq. — Dr. W. Rosenhain 

 made some remjarks in the same direction in /. Iron and Steel hist., 

 1Q06, II, p. 180 {see Metallurgies Vol. 8, p. 136, 1911). 



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