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 

 striking an Irregular Surface. 



Fig. 8. Path of Axial Light Rays 

 striking an Irregular Surface. 



yield diffraction patterns; for as shown in Fig. 8, the light rays 

 b 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 



