VERTICAL ILLUMINATORS, METALLURGICAL MICROSCOPES 79 



Nernst lamps with very small incandescent filaments often 

 fail to yield a sufficiently even illumination; under such con- 

 ditions a piece of ground glass interposed between lamp and illu- 

 minator will usually greatly improve the field of view, but will 

 of course reduce the brightness of image. 



To obtain satisfactory results in the study of opaque objects 

 with vertical illuminators it is important that the objectives em- 

 ployed be constructed with compact mounts and that the lenses 

 be corrected for use with uncovered objects. Standard microscope 

 objectives are always corrected for some definite cover glass 

 thickness. Moderate or high power objectives of this sort, 

 therefore, cannot be employed for the study of uncovered 

 preparations. 



Most objective manufacturers supply special objectives for 

 use with vertical illuminators. Such objectives have very short 

 mounts and have the rear lens combination flush with the upper 

 edge of the mount (see Figs. 36 and 46). This is done to prevent 

 internal reflections and yields better fields and clearer and 

 brighter images. It is a safe rule to follow, if the best results are 

 wanted, to select an outfit in which the distance between the re- 

 flecting surface of the illuminator and the rear lens combination 

 of the objective is as small as possible. 



The diagrams, Figs. 36 and 39, have been drawn with a view 

 of showing this in an exaggerated way. In Fig, 36 a short com- 

 pact mount is shown, the rear lens combination is almost in 

 contact with the reflecting prism P, while in Fig. 39 an ordinary 

 objective is shown and the distance between reflecting disk F and 

 the rear lens is so excessive as will doubtless lead to interfering 

 reflections of an aggravated sort. With the construction shown 

 in Fig. 39, an objective with compact mount would be essential. 



The interior walls of vertical illuminators must never be al- 

 lowed to become bright but must be kept coated at all times with 

 a dull black finish. 



Since the diameter of the rear lens combination is different in 

 different objectives, especially when manufactured by different 

 firms, it is evident that the best results will be obtained with 

 illuminators of the prism type, only when the prism can be dis- 



