66 PEACTICAL PHOTO-MICKOGEAPHY 



of fairly high power as there is obviously considerable loss 

 of light. 



For the illumination of opaque objects, where observa- 

 tion with high powers is required, recourse must be had to 

 either one of the two available types of vertical illuminators 



(Fig. 18 a and b). It is essential 

 when using these vertical illumina- 

 tors that the object should be 

 either in optical contact with the 

 cover-glass or should be un- 

 covered preferably the latter. It 

 is an important adjunct in metal- 

 lurgical work, and, in fact, as 



F,o.l7.-Reflecting Illuminator alread y mentioned, recent im- 



for Low Powers. provements in the illumination of 



opaque objects and in the method 



of obtaining variations of the beam of light incident on such 

 surfaces, has been almost entirely due to its use in the 

 examination of metal surfaces. 



a b 



FIG. 18. Vertical Illuminator for Opaque Objects. (W. Watson & Sons.) 



The vertical illuminator is attached to the microscope in the 

 position usually occupied by the objective, and the objective 

 is then screwed into its lower end. It therefore occupies an 

 intermediate position between the objective and the ocular, 

 but is situated as near as possible to the former. It consists 

 essentially of either a thin cover-glass, or, in some cases, of an 

 optically worked plane piece of glass ; or, as in the Zeiss vertical 

 illuminator, of a small totally reflecting prism. A beam of 

 light is projected at right angles to the optical axis of the 

 microscope so that it falls either on to the reflecting cover-glass 

 or on to the prism and is thrown down through the objective 



