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 merel}' 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 lUuminator 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 
under spring tension which serve to 
hold the lamp upon the objective. 
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 upo'n the objective and holds the illuminator securely in 
Handles 
Holder 
Fig. g. Silverman Illuminator. 
Lamp and Holder. 
