Elementary chemical microscopy elementarychemi00cham Year: 1921 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
Elementary chemical microscopy elementarychemi00cham Year: 1921 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 ^Handles 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 ringers Holder under spring tension which serve to i u ,i i ,i i • Fig. q. Silverman Illuminator, hold the lamp upon the objective. T , TT ,, r L J Lamp and 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. Rele
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