Improved Metallurgical Microscope. By Walter Rosenhain. 151 



the aperture of the objective, by far the best, giving decidedly better 

 results than the prism illuminator which is so much employed. 

 In order, however, to enable the observer to study the effects of 

 oblique lighting with high-power wide-angle objectives, a whole 

 series of reflectors of different shapes and sizes is provided for the 

 instrument ; these are illustrated in fig. 23. 



It has already been pointed out that the illuminator slide fits 

 into the dovetailed grooves provided outside each of the three 

 openings in the body tube, so that the Microscope can be placed in 

 any of the three positions relatively to the source of light ; once 

 the relative position of Microscope and illuminant has been 

 properly adjusted, no change will be required when objectives of 

 different foci, or specimens of different thickness, are used. Inter- 

 changeable with the illuminator slide, a series of other slides is 

 provided. One of these carries an iris diaphragm on a swinging 

 arm ; this is inserted on the side facing the illuminant, and serves 



Fig. 23. 



to stop down the incident beam, if required ; the use of the slide 

 and of the swinging arm making it possible to adjust the aperture 

 of the iris to the position found most desirable for the reflector of 

 the illuminator. The ring carrying this iris is also fitted with a 

 standard objective screw-thread, by means of which lenses may be 

 attached ; perhaps the most important use of a lens applied at this 

 point is the application of a negative lens for the purpose of pro- 

 ducing critical illumination. In the ordinary course the best 

 means of obtaining critical illumination for opaque objects, is to 

 employ a source of light of considerable area, such as an in- 

 candescent gas burner with an opal chimney, and to place this 

 source of light on a level with the aperture of the illuminator and 

 at a distance from the reflector of the illuminator equal to the 

 distance from the reflector to the back conjugate focus of the 

 objective. Under these circumstances, the image of the source is 

 formed by the objective itself upon the surface of the specimen. 

 The image of the source, as formed by the objective at the eye- 



