ILLUMINATION OF OBJECTS; ILLUMINATING DEVICES 33 



in polishing are somewhat less pronounced. On the other hand 

 if cracks, fissures, pits, etc., or cleavage lines or slip bands are 

 to be searched for, as for example, in badly strained alloys or 

 in the study of fatigue failures, illumination by means of very 

 oblique rays is unquestionably the procedure to be followed. 

 In studies of the latter sort the preparation should be rotated, 

 since when fine striations lie approximately parallel with the 

 direction from which the illuminating rays emanate they are 

 almost invisible, but if the preparation be turned so that the 

 direction of the striations or cleavage lines lies at right angles, 

 or nearly so, to the direction of the light rays, the striations 

 and lines become prominent. Advantage may be taken of this 

 phenomenon in the photography of specimens which are badly 

 scratched and in which some other prominent feature is to be 

 emphasized in the photograph. In such an event the prepa- 

 ration may be illuminated with oblique rays from a powerful 

 radiant and the specimen turned until the scratches practically 

 disappear. 



There are many objects and many types of investigation 

 where merely the surface illumination is sufficient and it matters 

 little whether the light rays are nor- 

 mal or oblique, under these conditions 

 the Silverman Illuminator is a great 

 convenience and yields excellent re- 

 sults. 



The Silverman Illuminator consists 

 of a single filament, tubular tungsten 

 lamp bent in the form of a circle. 

 The lamp is held in an annular mount- 

 ing provided with three curved fingers Holdei 



under spring tension which serve to 



i_ u ..i i ,1 i . FIG. o. Silverman Illumyiator. 



hold the lamp upon the objective. Holder. 



Fig. 9 shows the lamp in its mount- 

 ing. Pressing together the knurled heads H, H, forces back the 

 fingers and thus enlarges the opening for the passage of the 

 objective. Releasing the handles allows the fingers to press 

 tightly upon the objective and holds the illuminator securely in 



