264 PEACTICAL PHOTO-MICBOGKAPHY 



The illuminant is placed at a distance of about nine or ten 

 inches from the body-tube, and it will then generally be found 

 that the image of the illuminant is practically in focus when the 

 surface of the object itself is in focus. Any alteration in this 

 respect to ensure a more exact focussing of the illuminant 

 must be effected by altering the distance of the illuminant from 

 the microscope itself. Where a bull's-eye or other condenser 

 is interposed between the source of light and the microscope, 

 it is advisable to use a parallel or only slightly convergent 

 beam. The size of this beam should be cut down by an iris- 

 diaphragm CO (Fig. 70), so that no stray light is allowed to enter 

 the objective. The exact cutting down of the dimensions of 

 the beam is, perhaps, one of the most essential points to 

 attend to in metal-work, as it is obvious that with the highly 

 reflecting surfaces of metals, considerable diffusion of light 

 may easily occur. In the later forms of vertical illuminators 

 this is provided for so that not only the beam impinging 

 on the illuminator, but the actual size of the beam entering 

 the back combination of the objective may be regulated 

 at will. 



The range of magnification required is not so great as in 

 work with ordinary transparent objects ; in fact, observation 

 of published results shows that a great deal of it is done at 

 magnifications of either 100 or 1000 diameters. In the early 

 days of metallography it was thought that only low powers 

 were of use in this class of study, investigation by higher powers 

 having failed at that time to lead to any interesting results: 

 It was only when Mr. J. E. Stead took up the subject of high- 

 power work that it was found that a new order of structure 

 could be revealed. It has therefore become almost customary 

 either to treat the object from a low-power standpoint or at 

 once to go to much higher magnifications. The result is that a 

 large range of objectives will not in general be required. Achro- 

 matic or apochromatic objectives are available, and the same 

 general conditions apply as in ordinary work. The necessary 

 working N. A. should be as large as possible, since the loss of light 

 is very much greater than in work with transparent objects, 

 with the consequence that unless a high-power illuminant is 

 used the exposures become unduly lengthened. 



The time of exposure is not so difficult to estimate as 



