ILLUMINATION OF OBJECTS; ILLUMINATING DEVICES 31 
When, however, axial reflected light is used, that is, when the 
illuminating beam strikes the polished preparation normal to 
its surface, the plane surfaces will appear bright, the irregularities 
more or less dark, and minute projecting irregular points will 
Fig. 7 . Path of Oblique Light Rays Fig. 8 . Path of Axial Light Rays 
striking an Irregular Surface. striking an Irregular Surface. 
yield diffraction patterns; for as shown in Fig. 8, the light rays 
h and c, striking reflecting surfaces, are turned aside at such an 
angle as to preclude their entering the objective. 
Not infrequently a preparation yields an image consisting in 
part of a network of fine black irregular lines or of overlapping 
concentric black circles. It may then be very difficult to decide 
whether the preparation is actually marked with an intricate 
pattern or whether the reticulations seen in the image are merely 
the result of diffraction patterns. Rotating the preparation by 
turning the stage (or if the microscope has no rotating stage, 
turning the specimen) while looking into the microscope will 
usually greatly aid in clearing up perplexing problems of this 
sort. 
Careful consideration of the above described phenomena is 
absolutely essential to a correct interpretation of the structure 
of the material being studied. To determine when one is dealing 
with depressions and when with elevations when working with 
moderately high powers and vertical or oblique illumination is 
often a difficult problem which is further complicated for the 
beginner by the fact that the image seen is that of the object in 
a completely reversed position. 
An elevation as seen with the naked eye cast a shadow on the 
side away from the radiant, that is, the side of the elevation 
exposed to the source of light will be bright, the other side will 
