164 THE HIGH-POWER PHOTOMICROGRAPHY 



III. — The Reichert Microscope. 



This instrument, which is too well known to need description 

 here, has often been regarded as a convenient apparatus for use in 

 those industrial establishments where convenience and rapidity of 

 working are, perhaps, of somewhat greater importance than the very 

 finest definition and resolving power. An investigation recently made 

 by Professor Carl Benedicks and E. Waldow (Jern-Kontorets 

 Annaler, 1918, 537) has shown, however, that far from such being 

 the case, photographs of the very highest quality can be obtained. 

 Several micrographs published by these workers at 1,200 diams. leave 

 little or nothing to be desired. The perfection is further illustrated 

 by an enlargement at 3,500 diams. which constitutes one of the most 

 excellent photomicrographs of steel at such a magnification ever 

 obtained. This magnification, however, has only been used to show 

 what sort of effect is the best at present obtainable, and does not 

 represent one which will yield critical results. Around the globules 

 and laminae of FcaC are thick black halos up to and exceeding 

 0.5 mm. thick. 



The authors point out the much lower quality of picture obtained 

 when the prism vertical illuminator is used as compared with that 

 when the plain disc is chosen. ' This fact, first pointed out by 

 Professor Benedicks himself, is one of prime importance in connection 

 with high power microscopy. Since in one direction the prism cuts 

 off half of the back lens of the objective, the resolving power is 

 curtailed accordingly. To pay big prices for Zeiss lenses of high 

 N.A. and then deliberately to cut down by the prism illuminator the 

 resolving power to much less than that obtainable with a lens of 

 similar focal length at a fraction of the cost, argues a very poor 

 acquaintance with the theory of the instrument. Since this loss of 

 resolution operates chiefly in one direction, there is a tendency for a 

 circular object to be drawn out into an ellipse. Further, rows of 

 globules oriented in the direction of the longer axis of the 

 ellipse are much more likely than ever to be rendered as a continuous 

 band, while laminated structures maj' appear structureless or be 

 correctly resolved, according to the direction. Fig. 3. The marked 

 superiority of the disc should need no further emphasis. 



As one would expect, the strength of the source of light is shown 

 to be without effect on the quality of the photograph produced. When 

 vibration is considered, however, and this is rarely absent from 

 steel works laboratories, the much shorter exposure required with 

 powerful arc illumination, renders it then possible to obtain sharper 

 negatives than with weaker lighting. In connection with vibration 

 it is suggested that the objective should be separated from the body 

 tube, which will result in a considerable decrease of mass in those 

 parts specially sensitive to vibration, whereby the amplitude of the 

 latter will be reduced and non-recurring. 



While considering this point, the following is of importance. 

 Given a certain primary image, this can be magnified in two ways. In 

 the first place, it may be eye-pieced strongly, or alternatively, an eye- 

 piece of lower magnification can be used with a long camera extension. 

 Given absolutely rigid conditions there does not appear to be very much 

 to choose between the two methods, although perhaps the long camera 



